Improve Travis CI file caching

Travis CI VM image come with many preinstalled stuff for different
versions of scala and sbt. Optimize caching upload/download time by
storing only what is relevant for our build. Note that we assume that
the dependencies of the needed scala versions (e.g. 2.10.4 for sbt
0.13.6 and psp-std target version: 2.11.4) are both preinstalled. If
Travis CI team would not follow the upgrade pace, these artifacts could
also be cached (.sbt/boot/scala-X.Y.Z, ...).