latest alembic imcompatible with current sqlalchemy requirements

ceilometer migrations are failing in stable/havana build because
latest alembic release utilises code added in sqlalchemy>=0.9.2.
set upper bound for alembic to avoid issue.

Change-Id: Icc9e647d0c2ff418ead544fe671f3f2fe3c22da2
Closes-Bug: #1300105