[WIP] Compatibility with Rails 4.2
* add some testing dependencies to gemfile to avoid conflicts with system minitest
* fix errors for everything except most associations
* remove unused code (from before ar 4.1.5)
* move association_scope override code to match AR 4.2.0.beta1
* move has_many_association override code to match AR 4.2.0.beta1
* move has_many_through_association override code to match AR 4.2.0.beta1
* move preloader/association override code to match AR 4.2.0.beta1
oops typo
* fix has_attribute? to be sure it returns true when the attribute is an array
* fix 5 tests by adjusting relation#where_values_hash to match 4.2.0.beta1
* fix _update_records to match 4.2.0.beta1
* fix some more tests by only creating an array for associated record IDs when needed
fix some more tests by uncommenting the write_attribute for CPK attrs
* remove unused code
* fix setting array attrs to nil by setting each attr to nil
* do not use CPK when not needed, as it breaks polymorphic relationships here
* include error messages from non-mysql DBMSs as passable for the delete_all test
* set gemspec to depend on beta Rails so we can use it in other Bundler projects
* update uniqueness validator to match 4.2.0.beta1 and remove deprecation warnings
* do not use the PK for read_attribute when reading "id" for composite keys since this makes it impossible to read the "id" attribute of a CPK = [:id, :another_id]
* add a test to show we are caching something which ends up returning bad results