Space developer can provision a service instance with credentials for an existing external instance (a ProvidedServiceInstance) [Finishes #50700489]

-Also makes Object Serialization work for STI models
Break up shared examples for creating and updating

[#50700489]
Test refactor and improve logging

[#50700489]
Implement POST /v2/provided_service_instances

[#50700489]
added regression test for CC bug that broke CF

The inlined relations for managed service instances weren't showing the assocation urls.
Undo accidental bump to activesupport 4.0.0
Makes service plans public by default

[finishes #52321123]
Added the ruby-HEAD to the travis.yml
Added the latest ruby to the travis.yml
Fix regression in filtering a space's service plan
fix to_one relation URIs in JSON read/enumerate
Remove mock_class

Looks like it should have been removed in commit 6bc6dc1b2e9bb35373dc0e6
Merge pull request #58 from cloudfoundry/remove-dead-code

Remove mock_class.rb