Fix AR 4.2 support changes to be backwards compatible

This patch conditionally includes files to support using either
ActiveRecord 4.2 or older versions. When including Arel monkeypatches
it loads files based on the detected ActiveRecord version.

It also renames and combines the old visitor version to patch the
Arel PostgreSQL visitor instead of the base visitor class.

Finally it changes some tests to use Regexes that will match the
SQL produced by all supported versions of ActiveRecord.

An additional note: The version check in lib/postgres_ext/arel.rb will
need to be updated when the final version of ActiveRecord 4.2 is
released.