fix(closure): add Closure externs for angular.$q.Promise.finally

Closes angular#4757

Author: bigcmos

closure/angular.js

@@ -1667,6 +1667,12 @@ angular.$q.Promise;
  */
 angular.$q.Promise.then = function(successCallback, opt_errorCallback) {};
 
+/**
+ * @param {?function(?)} callback
+ * @return {angular.$q.Promise}
+ */
+angular.$q.Promise.finally = function(callback) {};
+
 /******************************************************************************
  * $route Service
  *****************************************************************************/