Remove index only if exists

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

db/migrate/20141121161704_add_identity_table.rb

@@ -14,7 +14,10 @@ SELECT provider, extern_uid, id FROM users
 WHERE provider IS NOT NULL
 eos
 
+    if index_exists?(:users, ["extern_uid", "provider"])
       remove_index :users, ["extern_uid", "provider"]
+    end
+
     remove_column :users, :extern_uid
     remove_column :users, :provider
   end
@@ -35,6 +38,9 @@ eos
       end
 
     drop_table :identities
+
+    unless index_exists?(:users, ["extern_uid", "provider"])
       add_index "users", ["extern_uid", "provider"], name: "index_users_on_extern_uid_and_provider", unique: true, using: :btree
     end
+  end
 end