Turn prepared statements back on

We once turned these off (in commit
abe71a767b) because they caused the
PostgreSQL query optimizer to use very suboptimal plans.  However,
PostgreSQL 9.2 has supposedly fixed this:

  http://www.postgresql.org/docs/9.2/static/release-9-2.html

So let's try again.

src/lib/Hydra/Model/DB.pm

@@ -20,8 +20,7 @@ sub getHydraDBPath {
 __PACKAGE__->config(
     schema_class => 'Hydra::Schema',
     connect_info => {
-        dsn => getHydraDBPath,
-        pg_server_prepare => 0,
+        dsn => getHydraDBPath
     },
 );