Change to https rubygems source

Using ":rubygems" results in the message:

  "The source :rubygems is deprecated because HTTP requests are
  insecure. Please change your source to 'https://rubygems.org' if
  possible, or 'http://rubygems.org' if not."
Add undeclared dependencies for thrift

thrift requires rack and thin in its Thrift::ThinHTTPServer but does
not declare them in the gemspec.
Merge pull request #58 from twitter/fix_travis_ci_build

Fix Travis CI build