Support for play-json 3.x

[matmannion]

The play-json team changed the groupId for the dependency from org.typesafe.play to org.playframework for 3.x and later (https://github.com/playframework/play-json/releases/tag/3.0.0).

As a workaround, it still uses the same package names so this library can be used by including the dependency directly and adding an exclusion rule, e.g.

libraryDependencies ++= Seq(
  "org.playframework" %% "play-json" % "3.0.1",
  "org.julienrf" %% "play-json-derived-codecs" % "10.1.0"
)

excludeDependencies ++= Seq(
  // As of Play 3.0, groupId has changed to org.playframework; exclude transitive dependencies to the old artifacts
  ExclusionRule(organization = "com.typesafe.play")
)

It may be worth marking the dependency as Optional in the library, to avoid pulling in incompatible play-json dependencies as various libraries update to support 3.x

[julienrf]

I think we should cut a new release where we update the dependency to play-json 3.x. Would you be interested in submitting a PR for that?

[matmannion]

No problem, will get that submitted today