Travis: Include running test suite with -Xexperimental in normal builds.

This will only recompile and rerun the test suite with the
flag enabled, rather than doing a full build from scratch just
to test this change.
Fix #423: Don't add non-js files to ScalaJSClasspath
Merge pull request #421 from sjrd/speedup-travis

Travis: Include running test suite with -Xexperimental in normal builds.
Merge pull request #424 from gzm0/fix-scalajsclasspath

Fix #423: Don't add non-js files to ScalaJSClasspath