Mongoid: Metadata accessor was renamed.

Since metadata was a common name in applications, we renamed it in
Mongoid 4 master to #relation_metadata.
Merge pull request #140 from durran/master

Mongoid: Metadata accessor was renamed.
Update Gemfiles to support multiple VMs. Remove .lock files.  Update specs to be RSpec 3.0 compatible.  Remove .ruby-version.  Add Ruby 2.1.0, JRuby, and Rubinius to Travis config.