Fix LBaaSv2 alembic migration for sqlalchemy 0.9+

sqlalchemy v0.9 does not support explicit Integer, BigInteger, and
SmallInteger sizes. Therefore, update the alembic script to support 0.9.
Merge pull request #4 from craigtracey/sqlalchemy0.9

Fix LBaaSv2 alembic migration for sqlalchemy 0.9+