Remove Gson, fixes #956

Along with replacing the usages of Gson this changeset also replaces direct
usage of Jackson's ObjectMapper with the matching methods from Play's Json
class.
Upgrade to Play 2.3.6
Sync JAX-RS and Jersey with dependencies in REST client
Remove unneeded Ebean ORM dependency
Add missing @BodyParser annotations

Reference for details and motivation: https://www.playframework.com/documentation/2.3.6/JavaBodyParsers