Add crossbulding for SBT 0.12 and 0.13

Requires this change in SCCT: https://github.com/SCCT/scct/pull/7

Cross publish for SBT 0.12 and 0.13 with the command `^publish`. See [sbt-cross-building](https://github.com/jrudolph/sbt-cross-building) for more details.

When testing locally I had to change `seq(ScctPlugin.instrumentSettings : _*)` to `seq(sbt.scct.ScctPlugin.instrumentSettings : _*)`