BM Migrations 2 & 3: Fix drop_column statements

`drop_column` takes a string not a `Column` object.

Change-Id: I01d537f78e40a20b762439087a13e104b9d18d46
Fix migration snake-walk.

The original code was not actually running `downgrade` because it
attempt to `downgrade` to the current version.

The fix is to downgrade to the current_version - 1.

Change-Id: I69484039269a17e343404bff195a6f84af56972c
Merge "BM Migrations 2 & 3: Fix drop_column statements"
Merge "Fix migration snake-walk."
Merge remote-tracking branch 'm_src_repo/master' into build-temp