Added dependency, it is not available on PyPI.

* Required as we are using 'flask.ext.alembic'
* See also https://github.com/tobiasandtobias/flask-alembic/issues/7
* Using the fork https://github.com/jace/flask-alembic
Merge pull request #68 from samyakbhuta/master

Added dependency 'flask-alembic', it is not available on PyPI.