Build for Scala 2.12.0

[adamnfish]

Hopefully this can be used to track 2.12 progress. The following dependencies are still missing builds for Scala 2.12, I've updated others where builds are now available.

• catalysts
• simulacrum
• spire

[travisbrown]

Both kind-projector and Simulacrum are ready, so here's the current checklist:

• Merge Fix Arbitrary instances in Free tests for ScalaCheck 1.13.3 #1451, which allows us to update to the most recent ScalaCheck (has two 👍s).
• Get the kind-projector, ScalaCheck, Scalaz, and Tut changes from my branch here into this PR.
• Wait for Scoverage to get published for 2.12.0 (this issue is I think the one to vote on).
• Update Scoverage in this Catalysts PR (which currently updates everything except Scoverage).
• Publish Catalysts 0.0.5.

[travisbrown]

The Scoverage compiler plugin and runtime are now out for 2.12.0 (thanks, @gslowikowski)! Now we're just waiting on the SBT plugin to be updated to Scoverage 1.3.0.

[travisbrown]

The changes here are incorporated into #1457, which handles the last remaining item on my checklist above.