Support Scala 2.12

[jongwook]

It can be done with almost no code change, as TRAVIS=true sbt ++2.12.1 test succeeds now.

One nuisance is that we would still want to use JDK 1.7 for Scala 2.10 and 2.11 builds for the maximum compatibility, whereas JDK 1.8 is required for 2.12.

We will need to relax our JDK 1.7 check, such that the check is only performed when SBT is publishing something.

I'm considering about using Jabba as a part of the build toolchain, but not sure if it will be a future-proof solution.