Fix #343: do not set scalaVersion in our sbt plugin.

Setting this dated back from the early days when only a
very specific version of Scala was supported. This does
not make sense anymore, and can be confusing if you set
a scalaVersion before importing scalaJSSettings in a
build file.
Merge pull request #348 from sjrd/dont-set-scalaversion

Fix #343: do not set scalaVersion in our sbt plugin.