Add missing OpenSSL cert generation parameter to .travis.yml.

 * (oonib)∃!isisⒶwintermute:~/code/torproject/ooni-backend ∴ date
    Sun Apr 21 03:18:17 UTC 2013
    (oonib)∃!isisⒶwintermute:~/code/torproject/ooni-backend ∴ travis-lint
    Hooray, /home/isis/code/torproject/ooni-backend/.travis.yml seems to be solid!