Add versions-maven-plugin 2.1.
Start development on 0.6.0.
Update retrofit to 1.6.0.

- https://github.com/square/retrofit/blob/master/CHANGELOG.md#version-160-2014-06-06
Add persistent API key for easier testing.
Support find endpoint.
Add language variants on person credits methods.
Add tv service credits method.
Merge branch 'find-and-tv-credits' into dev