Don't override the pip PI URI for TravisCI; use alternate ".requirements.travis".

 * (oonib)∃!isisⒶwintermute:~/code/torproject/ooni-backend ∴ date
   Sun Apr 21 03:29:49 UTC 2013
   (oonib)∃!isisⒶwintermute:~/code/torproject/ooni-backend ∴ travis-lint
   Hooray, /home/isis/code/torproject/ooni-backend/.travis.yml seems to be solid!