Bump .ruby-version to 2.1.2
Change the remove_global_http_status_column migration to be reversible

- Following me making this only a `change` migration in #319, a discussion
  happened in which it was decided that it is important that it is
  reversible, otherwise things break in the event of an aborted deploy
  if the column removed does not rollback to exist again.
Merge pull request #325 from alphagov/make_global_http_status_removal_migration_reversible

Change the remove_global_http_status_column migration to be reversible
Update type validation specs

Two tests using ensure_validation_of were still passing even though they
didn't have 'unresolved' in the array which should match the supported
types of mappings. Since they continue to pass as long as all elements
of the array the matcher receives are valid (even if the array is empty),
I think the clearest thing to do here is to just pass it Mapping::SUPPORTED_TYPES.
Merge pull request #326 from alphagov/update_validate_inclusion_specs

Update type validation specs
Merge pull request #324 from alphagov/upgrade-ruby-to-2.1.2

Bump .ruby-version to 2.1.2
Add hit_count to Mapping

* For precomputation to assist Sort By Hits
Micro-optimise Site#hit_total_count

* Get it in one hit, don't iterate rows
Make spec language consistent

* We only use "should" in assertions
Refactor Import::HitsMappingsRelations

* Instance it so we're not passing around site
* Remove some duplication around IN clauses
Precompute mapping hit counts

* Part of Import::HitsMappingsRelations.refresh!
* Not too onerous a query
Remove Mapping#with_hit_count

* expensive scope
* hit_count now precomputed and available as an attribute on Mapping