Default pattern shouldn't be empty.

If no `:pattern` option is passed, the default shouldn't be empty. It should match the `Guardfile` template.

Example of why this matters: http://stackoverflow.com/questions/34806877/how-do-i-get-guard-to-automatically-compile-my-coffeescript-file-says-it-is-but

Implementation: see https://github.com/guard/guard-coffeescript/blob/5a307f7/lib/guard/coffeescript.rb#L42
