Backport (#517) Add partest interface compile to travis

(cherry picked from commit 2cefff072ae8de907eb4ad9a4cb88e017bf5c3da)

Conflicts:

	.travis.yml
Backport (#517) Fix #538: Correct type signature for append methods on StringBuffer

(cherry picked from commit c16793874c88ac3e185c612715ecfb3585d66639)

Conflicts:

	test/src/test/scala/scala/scalajs/test/javalib/StringBufferTest.scala