Add alembic to requirements

gertty requires it now but does not declare that.

Also, move pbr to the top of the requirements list. Because of how
python requirements and transitive depends, pbr needs to be listed
first or else hijinks can ensue.

Change-Id: I06c14039501c239d6ead1a66ce8fcfaa7453855c

requirements.txt

@@ -1,7 +1,9 @@
+pbr>=0.6,!=0.7,<1.0
+
 urwid
 sqlalchemy
 GitPython>=0.3.2.RC1
 python-dateutil
 requests
 ordereddict
-pbr>=0.6,!=0.7,<1.0
+alembic>=0.4.1