Fix failing migrations when running rake db:migrate task

This patch adds 2 missing colons (:) for 2 migrations that are failing when
the `rake db:migrate` task is ran. Whilst already-committed-to-source-control-
and-ran-on-production-environments migrations shouldn't be modified whatsoever,
this change does not modify in any way the current database schema so, technically,
it shouldn't affect forks that might have ran these migrations already and those
CONSUL installations that haven't done so should be able to without problems

db/migrate/20170915101519_add_legislation_proposals_count_to_tags.rb

@@ -1,6 +1,6 @@
 class AddLegislationProposalsCountToTags < ActiveRecord::Migration
   def change
     add_column :tags, "legislation/proposals_count", :integer, default: 0
-    add_index :tags, "legislation/proposals_count"
+    add_index :tags, :"legislation/proposals_count"
   end
 end

db/migrate/20170922101247_add_legislation_processes_count_to_tags.rb

@@ -1,6 +1,6 @@
 class AddLegislationProcessesCountToTags < ActiveRecord::Migration
   def change
     add_column :tags, "legislation/processes_count", :integer, default: 0
-    add_index :tags, "legislation/processes_count"
+    add_index :tags, :"legislation/processes_count"
   end
 end