Adds Play 2.8.x support, drops Scala 2.10 support

[a1kemist]

In play-json 2.8.x, the writes trait was changed from Writes[-T] (contravariant) to Writes[T] (invariant) which breaks enumeratum's PlayJsonEnum trait. This PR does the following:

• Adds an implicit helper to contraMap the Writes provided by the existing trait. This seems to be sufficient enough to support play-json 2.8.x without any more complicated code changes.
• Drops Scala 2.10 support. While working on this PR there were a few sporadic dependency resolution issues with resolving 2.10 based dependencies, but now I'm unable to resolve any of the 2.10 dependencies when building the project, so I've elected to drop 2.10 support with this PR.
• Cleans up build.sbt now that it seems like Scala 2.13 is supported on all projects.

[coveralls]

Coverage increased (+0.02%) to 90.726% when pulling f29cc93 on a1kemist:play-2.8-support into 9c41215 on lloydmeta:master.

[lloydmeta]

❤️ the cleanup in the build, thanks !

Also +1 to dropping 2.10 support at this point :)

Going to merge this tmw when I am back.

[lloydmeta]

Thanks for this @a1kemist, and apologies for releasing later than I said. Just merged, and pushed a new release. It should be up on Maven Central in a bit.