pin selenium-server for CI compatibility

package.json

@@ -62,7 +62,7 @@
     "rollup-plugin-buble": "^0.15.0",
     "rollup-plugin-replace": "^1.1.1",
     "rollup-watch": "^3.2.2",
-    "selenium-server": "^3.0.1",
+    "selenium-server": "^2.53.1",
     "todomvc-app-css": "^2.0.6",
     "typescript": "^2.1.5",
     "uglify-js": "^2.7.5",