Make default build profile optional

In my project, I don't want the default build profile to get run as I'm just using require to build the modules with almond.js

This PR allows you to set ```REQUIRE_BUILD_PROFILE=False``` to disable the default profile


The same as this commit, with docs and a test case: https://github.com/DjangoAdminHackers/django-require/commit/2e2f52014850b4063f14d90fa0c9914897fcf608