ActiveRecord 4.2

This is a work in progress. Tests are not yet passing. Help is welcome.

There are significant changes to the PostgreSQL adapter in ActiveRecord 4.2, and it is extremely cumbersome to support prior versions in a single code base. Therefore, instead of having conditional support for prior versions of ActiveRecord, this adapter will only support ActiveRecord 4.2 and later in the master branch and version 3.0.

`activerecord-postgis-adapter` version 2.x will continue to support ActiveRecord 4.0 and 4.1.

`activerecord-postgis-adapter` version 3.x will support ActiveRecord 4.2 and later.