Making sure pip-grep runs in the project directory

[cesarkawakami]

Running pip-grep outside the project directory causes problems for projects with direct dependency references in the same file system. E.g., a requirements.txt file like this:

-e <some subdirectory>

Is currently failing because the numpy-scipy script invokes pip-grep inside the .heroku directory, instead of inside the project directory itself.

This PR changes the behavior so pip-grep is called from the project directory. Hopefully this shouldn't change the behavior in any other way.