pin dependencies

Dependencies should be pinned as in [requirements.txt](https://github.com/openstack/sqlalchemy-migrate/blob/0.9.4/requirements.txt).

I'm working on a recipe for buildbot, and I have this kind of errors during tests at runtime:

```
$ buildbot create-master master

Traceback (most recent call last):
File "/home/dfroger/miniconda3/conda-bld/buildbot_1495624299918/_t_env/lib/python3.6/site-packages/pkg_resources/__init__.py", line 655, in _build_master
ws.require(__requires__)
File "/home/dfroger/miniconda3/conda-bld/buildbot_1495624299918/_t_env/lib/python3.6/site-packages/pkg_resources/__init__.py", line 963, in require
needed = self.resolve(parse_requirements(requirements))
File "/home/dfroger/miniconda3/conda-bld/buildbot_1495624299918/_t_env/lib/python3.6/site-packages/pkg_resources/__init__.py", line 854, in resolve
raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (pbr 3.0.0 (/home/dfroger/miniconda3/conda-bld/buildbot_1495624299918/_t_env/lib/python3.6/site-packages), Requirement.parse('pbr<1.0,>=0.5.21'), {'sqlalchemy-migrate'})

```

(edit: PR coming...)