When running tests, clear out temp files from the dummy Rails app

So that new assets are generated, even if the files themselves haven’t
changed.

If we change how assets are generated, we need to know if the generated
files are good or not in our integration test. However, since we
probably haven’t changed the actual files, new assets will not be
generated. Clearing out the temp files guarantees we’ll get new assets.
Merge branch 'rake'