Switch to SBT 0.13 (fixes #7)

I just updated the version of sbt-cross-building, the other work was
already done; this seems to work now (tested with `sbt '^publishLocal'`,
which uses cross-building).

This does not fix the deprecation warnings, but it's not clear the
project would still compile with other SBTs.
Merge pull request #11 from Blaisorblade/sbt0.13-2

Switch to SBT 0.13 (fixes #7)