Specify ruby version in Gemfile

[sauloperez]

This will make it easy to spot when someone is using a ruby version other than the one specified in .ruby-version. AFAIK, that doesn't stop devs from using another version when installing gems but it will update the Gemfile.lock with the version used.

Those that use rbenv, rvm or chruby will benefit from .ruby-version given that these version managers read it.