Fix two migrations to satisfy the Migration/AddConcurrentIndex cop

db/migrate/20170207150212_add_indexes_to_mirrors.rb

@@ -11,7 +11,7 @@ class AddIndexesToMirrors < ActiveRecord::Migration
   end
 
   def down
-    remove_index :projects, [:sync_time]
-    remove_index :remote_mirrors, [:sync_time]
+    remove_index :projects, [:sync_time] if index_exists? :projects, [:sync_time]
+    remove_index :remote_mirrors, [:sync_time] if index_exists? :remote_mirrors, [:sync_time]
   end
 end

db/migrate/20170208144550_add_index_to_mirrors_last_update_at_fields.rb

@@ -11,7 +11,7 @@ class AddIndexToMirrorsLastUpdateAtFields < ActiveRecord::Migration
   end
 
   def down
-    remove_index :projects, :mirror_last_successful_update_at
-    remove_index :remote_mirrors, :last_successful_update_at
+    remove_index :projects, :mirror_last_successful_update_at if index_exists? :projects, :mirror_last_successful_update_at
+    remove_index :remote_mirrors, :last_successful_update_at if index_exists? :remote_mirrors, :last_successful_update_at
   end
 end