add capabilities to define the ruby version in Gemfile by RUBY_VERSION or use 2.0.0 by default
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 ]
fix test-queue with mongoid 3 [ci-skip]
avoid some failure on within to short in test between date