Add local dependencies

PrettyTable 0.7.2
six 1.7.3 (as a wheel)

Change-Id: Ic71e3043c7db4347703c20629467b572f84a8205
Merge 'wikimedia/dependencies' into wm-master

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

Change-Id: I127bb119e8ce9f8ffa4b68411914c974fb803ab2