Specify ruby 1.9.3 in the Gemfile.

Heroku now uses ruby 2.0.0 as the default, but we appear to have some compatibilty issues somewhere (see https://github.com/hotsh/rstat.us/issues/772).

Heroku will use the ruby specified in the Gemfile, so this should make for a more seamless deploy experience until we fix the compatibility problem(s).