Be sure a secret_token is define in RAILS_GROUP == 'assets'

This case is need to avoid failure on rake assets:precompile if no ENV
are define like in heroku

See #461
Add ruby-head to test on travis-ci

Avoid define the RUBY_VERSION in Gemfile if CI
Add information about the Ruby version to use
Add information about RUBY version in Docs ENV too

[ ci-skip ]