Stop using pgcrypto extension

Now that we've got rid of path_hash entirely, we don't need this
extension. Even though there's a migration here to drop it, we need
to remove the extension from Puppet and make sure that it is dropped
in all environments by hand before we deploy this. Otherwise the
migration will fail because the transition Postgres role isn't a
superuser. We had a migration to enable this extension, so should
have one to remove it too, even though it isn't much use to us.
Merge pull request #470 from alphagov/pgcrypto-no-no-no

Stop using pgcrypto extension
SKETCH: Renormalise - add host_path_id to hits

* Drop other hits fields that are served by this
* Make the specs pass
* Oh my word import is blocking-slow. Now to optimise.
Optimise hits import UPDATE_FROM_STAGING

* Goes from blocking 3.5mins to about a second for 2000 hits_staging rows in dev
* Still slower than non-normalised on rake benchmark:import:hits at around 20s
* Try INSERTs next
Fix benchmark to acknowledge host_path

* Hits don't point to hosts any more