Bump alembic to 0.6.4 for Neutron

Recently merged neutron healing migration
(https://review.openstack.org/#/c/96438/)
needs alembic >= 0.6.4 for mysql.

The relevant fix in alembic:
    https://bitbucket.org/zzzeek/alembic/commits/b29780719904

Change-Id: If67241592ba82aed3b41b4acec17bd8d604a3c4b
Partial-Bug: #1342507
Merge "Bump alembic to 0.6.4 for Neutron"