Add Code Climate Coverage and Quality Badges

[NullVoxPopuli]

https://codeclimate.com/github/rails-api/active_model_serializers/code

Currently, they display as:

So, someone with proper permissions on the rails-api org will need to set up the coverage reporting.

[joaomdmoura]

Thanks @NullVoxPopuli we will do it for sure!

[bf4]

I can add the codeclimate simplecov formatter to #1069 (the pr that introduces simplecov)

[bf4]

Done in 02470bb but I didn't add the badges

[joaomdmoura]

Done.