Bump to sqlalchemy-migrate 0.8.2

This allows us to drop some unsightly sqlalchemy-migrate 0.7.3
patching.

The related requirements change is here:

  Icde6248ca102884323d7912bab0f4a716aa294ec

Change-Id: I5f80dfc07f8256716d025002fc1a1f47e58b2142
Merge "Bump to sqlalchemy-migrate 0.8.2"