mpich2: update to MPICH 3.0.1 and git

MPICH2 recently renamed itself to MPICH and bumped the version number
from 1.5 to 3.0.  Even more recently, it switched from SVN to git for
version control.

The sbin hack is no longer needed, since MPE was removed between
MPICH2-1.5 and MPICH-3.0.

This pull request is just a rebased version of the pull request from mxcl/homebrew#17019.  There was no problem in the formula, just a problem in the `autogen.sh` script on the MPICH side.