TypeError: superclass mismatch for class PostgreSQLAdapter

We shouldn't override PostgreSQLAdapter's superclass inheritance while monkeypatching. Changing Inheritance section leads to following error on JRuby

    TypeError: superclass mismatch for class PostgreSQLAdapter

After this change,
**Jruby**: The above error no longer appear. Tests are still failing but they do run. Progress!
**MRI**: No change, all green(tested by running `rake test_postgresql`) But I don't want to break MRI while trying to fix jruby. so lets  wait for travis CI build to run all tests to before we merge this in.

Thanks to @asanghi pairing with me on this. 

#11700

cc: @arunagw, @guilleiguaran