Conditionally remove indices that may not exist in pre-GitLab v6.7 installations

Closes #1593

db/migrate/20150425164648_add_missing_unique_indices.acts_as_taggable_on_engine.rb

@@ -3,8 +3,15 @@ class AddMissingUniqueIndices < ActiveRecord::Migration
   def self.up
     add_index :tags, :name, unique: true
 
+    # pre-GitLab v6.7.0 may not have these indices since there were no
+    # migrations for them
+    if index_exists?(:taggings, :tag_id)
       remove_index :taggings, :tag_id
+    end
+
+    if index_exists?(:taggings, [:taggable_id, :taggable_type, :context])
       remove_index :taggings, [:taggable_id, :taggable_type, :context]
+    end
     add_index :taggings,
               [:tag_id, :taggable_id, :taggable_type, :context, :tagger_id, :tagger_type],
               unique: true, name: 'taggings_idx'