Split out separate adapters for AR 3 and AR 4

Detect the AR version and load different adapters
approrpriately. Shared code (which is most of it) is still in
`ActiveRecordAdapter`, though that's been converted into a module so
that both the adapters could remain direct subclasses of
AbstractAdapter (not sure if that was a good idea or not, but it seems
to be working).
Add rails 4 and 4.1 to travis
Add tests for more complicated table structure
Add some excludes to travis

ree (or at least this version), 1.9.2, and 1.8.7 are not supported by
rails 4.1
Update changelog
[ci skip]
Merge pull request #97 from CanCanCommunity/rails4-adapter

Rails4 adapter