Change source in Gemfile to use http, not https.

Output from Travis-15, when trying to run `bundle exec rake
server:start` on heroku:

Could not load OpenSSL.
You must recompile Ruby with OpenSSL support or change the sources in your
Gemfile from 'https' to 'http'. Instructions for compiling with OpenSSL using
RVM are available at http://rvm.io/packages/openssl.

[GH-2]