re-add play2-core

[SethTisue]

fixes #272

[SethTisue]

green locally. let's see what Jenkins thinks https://scala-ci.typesafe.com/job/scala-2.12.x-integrate-community-build/628/consoleFull

I specified a newer Scala nightly (2.12.0-6df14e5-nightly) that everything isn't cached for yet, so it'll take a while

[SethTisue]

this is kinda janky stuff — we're using a commit from my wip branch in the playframework repo, forked from @schmitch's wip branch — but for Scala 2.12.0 to go forward, it's good enough. let's revisit this in a community build context once all the Play/2.12 issues have been ironed out in the normal Play build

[schmitch]

I actually rebased my PR against master, so If the build succeeds I try to get it included so that we can use the master branch: playframework/playframework#6432

Also be aware that the failed tests were in: Play-WS and Play-Integration-Test and I never run the documentation tests (which also tests stuff that won't happen in the Integration-Tests) against 2.12.x.