Merge branch 'ar_4.2.x' of git://github.com/codeodor/composite_primary_keys into codeodor-ar_4.2.x

Conflicts:
	composite_primary_keys.gemspec
	lib/composite_primary_keys.rb
	lib/composite_primary_keys/active_model/dirty.rb
Merge pull request #245 from codeodor/ar_4.2.x

Pull latest changes from master into 4.2.x branch
Merge pull request #255 from codeodor/ar_4.2.x

Merge master into Rails 4.2 branch and fix 1 broken test as a result.
Update to support Rails 4.2.0.rc2 (fixes #262)