Update polymer (#1921)

package.json

@@ -122,7 +122,6 @@
     "gulp-merge-json": "^1.3.1",
     "gulp-rename": "^1.4.0",
     "html-loader": "^0.5.5",
-    "html-minifier": "^3.5.20",
     "html-webpack-plugin": "^3.2.0",
     "husky": "^1.1.0",
     "lint-staged": "^7.3.0",
@@ -154,7 +153,7 @@
     "samsam": "1.1.3",
     "supports-color": "3.1.2",
     "type-detect": "1.0.0",
-    "@polymer/polymer": "3.0.5",
+    "@polymer/polymer": "3.1.0",
     "@webcomponents/webcomponentsjs": "2.1.3",
     "@webcomponents/shadycss": "^1.5.2",
     "@vaadin/vaadin-overlay": "3.2.0-alpha3",