Updates JSON API Adapter to generate RC4 schema

[benedikt]

As JSON API RC4 was published yesterday (with the final release date pushed back to 2015-05-28), this pull request updates the JsonApi adapter to match the new changes to the schema.

Relevant changes to the spec:

• Attributes (other than id and type) are now nested under the attributes key.
• Relationships are now nested under the relationships key instead of the links key
• The linkage key for relationships is now called data

This resulted in these changes to the gem:

• Tests are adjusted to reflect the new schema
• The adapter now renders the new schema
• The methods releated to links are renamed to avoid confusion
• fragment_cache now does a deep_merge instead of just a simple merge

Feedback welcome! I'm happy to adjust things as necessary. :)

[joaomdmoura]

Hey @benedikt, tks for submit this!
I'll review the PR and get back to you 😄

[jamesbrooks]

👍

[kurko]

Awesome!