[ci skip] bump to 3.3.1

Author: dfreedm

lib/mixins/element-mixin.js

@@ -24,7 +24,7 @@ import { wrap } from '../utils/wrap.js';
  * Current Polymer version in Semver notation.
  * @type {string} Semver notation of the current version of Polymer.
  */
-export const version = '3.3.0';
+export const version = '3.3.1';
 
 const builtCSS = window.ShadyCSS && window.ShadyCSS['cssBuild'];