port to OSX and Ubuntu/trusty

This is a bunch of little fixes to make sure fdroidserver works on OSX and Ubuntu/trusty. OSX since 30% of Android devs are on OSX. Ubuntu/trusty since Windows Subsystem for Linux, Travis-CI and many other things run Ubuntu/trusty. This overhauls .travis.yml which runs when manually pushed to https://github.com/fdroidtravis/fdroidserver and is available at https://travis-ci.org/fdroidtravis/fdroidserver

One nice side benefit of this work is that fdroidserver now works with files that have spaces in them. Or at least the test suite does, there might still be some places where spaces cause problems.