Fix #323: AssertionError when running scaladoc.

PrepJSExports referred to phase 'uncurry' for its behavior, but
that phase does not exist under scaladoc. Since the given
behavior is not essential for generating documentation, it is
simply disabled if forScaladoc is true.
Add packageDoc to the Travis build.

So that issues like #323 do not come back haunting us.
Merge pull request #328 from sjrd/fix-publishing

Fix #323: AssertionError when running scaladoc.