Require greenlet 0.3.2 (or later)

Version 0.3.2 resolves a bug that allowed generic "except
Exception:"
clauses to catch GreenletExit exceptions.

bug 1097203

Change-Id: I5b8872088cd9ed8fc0bcca493833c11884e349e3
Merge "Require greenlet 0.3.2 (or later)"