Test against Ruby head on travis
Update README [ci skip]

Currently the following builds are broken:

  * any builds against Ruby 2.2.0
  * AR edge builds
Update gemfile for ActiveRecord ~> 4.1.0

It's already released.
Do not lock the version of sqlite3 if Ruby is 2.1.1 or higher
Remove :rbx in favor of 55d316f396a38ec577ac727e8c0515ff2433c648
Allow specs against ruby-head to fail