Revert this downgrade stuff -- it's not necessary.

This reverts commits c7acf5650ce99c50611dce701e7f8f697f7290d3 and
9027dbf294872dcced963e0eb05112039e7c02e5.
Lock PostgreSQL to 9.1.6 in production, 9.1.8 elsewhere.

This also contains a fix for [1] that lets us try out 9.1.8 in e.g.
staging.  Hopefully said fix can be taken out soon; I'm not sure if the
scope of the fix is too large.

[1]: http://permalink.gmane.org/gmane.comp.db.postgresql.bugs/31876