Investigate SASS compilation warnings

[thorncp]

Log output from a Heroku deploy:

Running: rake assets:precompile
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span1" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span2" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span3" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span4" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span5" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span6" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span7" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span8" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span9" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span10" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span11" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
WARNING on line 4 of /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/app/assets/stylesheets/beach/beach.css.scss: ".span12" failed to @extend "[class^="span"]".
The selector "[class^="span"]" was not found.
This will be an error in future releases of Sass.
Use "@extend [class^="span"] !optional" if the extend should be able to fail.
I, [2014-05-27T18:40:42.617775 #647]  INFO -- : Writing /tmp/build_5942f557-e265-4f33-b846-6c4e6a1ac921/public/assets/application-dad118d806ea0c2c1d18d2d4446e64cb.css
Asset precompilation completed (9.71s)