Stop using our fork of paver and use PyPI

The problem we were having with paver 1.2.4 has been fixed and so we can go back to using the mainline paver now.

requirements/edx/paver.txt

 # Requirements to run and test Paver
-git+https://github.com/jzoldak/paver.git@b72ccd7b638c1e07105d04f670170b3a37095d10#egg=Paver==1.2.4a
+paver==1.3.4
 libsass==0.10.0
 markupsafe
 -r base_common.txt