Add Rails 4 compatibility under JRuby

This adds Rails 4 compatibility under JRuby and the activerecord-jdbc-adapter. This addresses https://github.com/dazuma/activerecord-postgis-adapter/issues/76. Fixing this mostly involved making the adapter skip the OID usage stuff, since the jdbc adapter doesn't utilize that.

This also fixes an issue I discovered while running the test suite: Rails 3.1 compatibility was broken under JRuby if you were using the activerecord-jdbc-adapter 1.3.x (but it still worked fine under 1.2.x). I'm not sure how many people are actually using this likely obscure combination of versions, but the fix was relatively straightforward, so I fixed that too.

Note that the Rails 4 tests will currently fail on Travis due to this rgeo-activerecord issue: https://github.com/dazuma/rgeo-activerecord/pull/12 But that issues affects all ruby platforms under Rails 4, not just JRuby. So I don't think you'll get clean tests until that issue is addressed in rgeo-activerecord, but in my own local testing, I get clean tests when building against that rgeo-activerecord branch.