Fix HitsMappingsRelations import

* Straight mySQL -> postgreSQL swap
* INSERT IGNORE simulation and ANSI update style
Fix lack of type conversions

* A couple of dynamic columns used for counts on model scopes
  (usually SUM(something) as attr_name) were coming back as strings
  see https://groups.google.com/forum/#!topic/ruby-pg/egzlYNi2OKk)
* Lay a method on top of the attribute to do the conversion on the fly
* Better suggestions welcomed!
Fix mappings from host paths

* We really need the GROUP BY, and the way we tell Postgres that
  the path is effectively represented by the hash is to use MIN
  as the aggregate. We can no longer use #pluck as this will affect the
  SELECTed field, but we can get the same behaviour by simple #map.
Fix hit scope groupings

* Fall back on MIN for path_hash, mapping_id and host_id when grouping hits