Revert "Describe SQLAlchemy constraint more clearly."

<0.8 matches any beta release of 0.8. We however want 0.7.x.

This reverts commit 2dc005e4f95fe3c4dd00fdc14c74f99bb791a3d7.

Change-Id: Iab46f560ca512cdd6b68b6cf0f7e52e530c9425f