Support both activerecord 4.1 and below

Still need to update specs to run on activerecord <= 4.0
Bump rspec to 2.14.0
Separate specs per activerecord version
Remove arel from Gemfile

Not really sure why it was there, activerecord seems to load arel
already. Plus it was giving some conflicts when running the build on
rails versions other than master (you needed to define both RAILS and
AREL env vars)