Fix failing Travis builds: Travis pants bootstrap

I found that Travis CI jobs were failing on `master`.

I was able to fix this primarily by adding a call to `pants.bootstrap`.

I also had to update a pytest version and remove a line that tests Java and Scala stuff. This is probably not the right solution (well I would love if it if pants and pex were in a separate Python-only repo instead of a monolithic repo...) but hopefully it spurs someone to fix it properly.