requirements: bump pygit2
requirements: bump pygit2

Pretty much replicates what you did last time in this commit:
https://github.com/PressLabs/gitfs/commit/8a53f6ba5ce2a4497779077a9249e7b4b5fcc32b

As far as my tests go, everything functions as expected, but the tests
were limited to importing gitfs, importing pygit2 and then creating
an empty repo using pygit2, so you may wish to put it to more
thorough tests yourself.

PressLabs/gitfs,ksmaheshkumar/gitfs,bussiere/gitfs,rowhit/gitfs,PressLabs/gitfs