= build: update to sbt 0.12.4-RC3

Performance of (cold) sbt compilation for Scala != 2.9.2 was slow. This
should improve the speed of travis builds.

See
https://groups.google.com/forum/#!topic/simple-build-tool/HMjm3jpHc6Y
Merge pull request #343 from jrudolph/wip/upgrade-to-sbt-0.12.3-RC3

= build: update to sbt 0.12.4-RC3