overall coverage and quality of 2.12 build leaves much to be desired

[SethTisue]

around the time of 2.12.0-M2, we threw together a 2.12 build by-any-means-necessary. it's pretty janky. I've opened tickets on many of the most significant issues (frozen sbt, forked Play, frozen scala-js, commented-out ENSIME, and so on and so on), but there plenty more forked and frozen projects and projects where we aren't running the tests.

I believe we've already overspent our notional budget for 2015 for doing big, intensive rounds of upgrades here, but it would be good to keep chipping away at this.

[SethTisue]

now that RC1 is imminent it's probably time to put some concerted effort into this

[SethTisue]

great strides on this made in #324 (as well as other, smaller recent PRs)

some things I'd like to at least take a stab at before closing this ticket:

• ScalaCheck upgrade (we're still on 1.11.x)
• uncomment spray (too hard right now, details at re-enable Spray test suite #110)

there's always lots of things that could be improved, but those seem like the big two

[SethTisue]

the ScalaCheck upgrade is #13, so I think we can close this one now.