upgrade to sbt 0.13.1-RC2
upgrade to sbt 0.13.1-RC3
avoid deprecated method

I think it was already deprecated in 0.13.0, but sbt 0.13.1 now
applies compile settings from /project/*.sbt to *.sbt files as well as
*.scala files, so we are now being notified of the problem for the
first time
make various sbt-based scripts quieter on startup

using --warn flag which is new in sbt 0.13.1
redo project/LanguageTests.scala in a simpler way

using the very useful new toTask method in sbt 0.13.1, plus
the inputTaskDyn macro introduced in 0.13.0

I couldn't figure out how to make only the language tests
have logBuffered false, so I went ahead and made it universal
for testOnly