Rails 4 knows about Postgres extensions

* So use the idiomatic migration commands (taking advantage of the fact
  there's non-dev PG DB at time of writing to re-use the original migration)
* Remove the now-redundant explicit SQL support for specs

**NB** there's more in the schema diff than you might expect; Rails 4 also changed the default option hash style from `:hashrocket => 'thing'` to `not_a_hashrocket: 'thing'`