Workbench incompatible with Play 2.7.0

[mgilbertnz]

I've just upgraded from Play 2.6.20 to Play 2.7.0 and have noticed an incompatible version of Akka when running startWorkbenchServer.

sbt:admin> startWorkbenchServer
Detected java.lang.NoSuchMethodError error, which MAY be caused by incompatible Akka versions on the classpath. Please note that a given Akka version MUST be the same across all modules of Akka that you are using, e.g. if you use akka-actor [2.5.18 (resolved from current classpath)] all other core Akka modules MUST be of the same version. External projects like Alpakka, Persistence plugins or Akka HTTP etc. have their own version numbers - please make sure you're using a compatible set of libraries.                 

As a test I cloned PR https://github.com/lihaoyi/workbench/pull/49 that updated Akka and other libraries and ran publishLocal. I'm pleased to announce that this pull request fixed the issue for me.

Could we please get his merged so others can update to Play 2.7.0?

[mgilbertnz]

I've further updated the dependencies in PR #51