Build against Play 2.7.1 because of binary incompatibility

[mkurz]

@cchantep Can you please publish a new release? Thanks!

See https://blog.playframework.com/play-2-7-1-released/

Play-JSON also has a breaking change. It was depending scala-collection-compat, but this library does not guarantees binary compatibility at this point, so we removed the dependency and copy some relevant code inside Play-JSON instead. If your application is using scala-collection-compat as a transitive dependency, or either if it uses a library that uses Play-JSON, these libraries will need to adapt to this change.

I already ran into this problem here: playframework/play-json#259