Add "_test" suffix to JsonApi::HasManyTest filename.

"rake test" was previously skipping this file because it didn't have the
proper suffix.
Merge pull request #653 from alexgenco/test_path_fix

Add "_test" suffix to JsonApi::HasManyTest filename.
Require only activemodel instead of full rails
Merge pull request #667 from ride/require-only-activemodel

Require only activemodel instead of full rails