Switch six from wheel to tar.gz

pip on Precise apparently does not support it or need some other python
dependencies to interprete the wheel.

Change-Id: I9ee70eededa2b0cc139bd38a0a39f6d4fe55336f
Merge branch 'dependencies' into master

Adjust requirements.txt to point to the tarball of PrettyTable and six.

Change-Id: I7c76c76f5b2e213b7ea71a3ab019cb7a895b50a8