travis: attempt to fix unlikely build issue

This is a long chain of errors, and there are likely other changes that could
be made to cope in other places along that chain.

If depends don't build successfully, don't bother trying again for the sake of
better logging. That's likely to hurt more than help. In this case, qt build
failed, and on the second attempt, it appeared to be successful. However, due
to a bad object from an internal gcc error on the first build, the resulting
lib was unusable. This caused bitcoin-qt to not be built, and tests and
packaging which expected bitcoin-qt to be there failed.

The root cause:
Mingw is especially crashy when using -jX, likely compounded by low-memory
environments. I've seen multiple problems with this combo in Gitian as well.

In this case:
i686-w64-mingw32-g++: internal compiler error: Killed (program cc1plus)
...
make[3]: *** [.obj/release/qdrawhelper.o] Error 4

The workaround:
Bump Travis down to using -j2 by default. Additionaly, enable --with-gui for
the windows builds. This will cause configure to fail if qt is not working
while also testing the config flag.

Other failures which may be worth revisiting separately:
- If a depends package fails, maybe remove the workdir so that it doesn't taint
  subsequent runs
- See if there's anything repeatable about the ICE when building qt
depends: cleanup better after qt and force a bump

qt needs to be rebuilt for travis. The previous commit should help ensure that
this won't need to be done again.
travis: temporarily disable the forknotify test

It appears to be breaking randomly on Windows