bugfix and parallel upgrade search

[tkruse]

Hi, this is my first pull request ever, so please correct me if I did something wrong.

one commit just makes sure get_highest_version does what it should (unit test added)

last commit makes -U be mass-parallel, taking much less time that way.

[cakebread]

Thanks, this looks great. I'm working on a new release, going to pull it in and try it.

[tkruse]

cool. Also got the error caught in the added commit, for a package installed via Ubuntu apt-get

In general I also have issues running
pip install -U yolk -U | awk '{print $1}'

because of packages installed in ubuntu via apt-get. Not sure whether that can be fixed.