Update polyserve requirement

Updates the requirements on [polyserve](https://github.com/Polymer/tools) to permit the latest version.
- [Release notes](https://github.com/Polymer/tools/releases)
- [Commits](https://github.com/Polymer/tools/commits/polyserve@0.27.13

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

tests/ui-regression/package.json

@@ -15,7 +15,7 @@
         "pixelmatch": "^4.0.2",
         "png-js": "^0.1.1",
         "pngjs2": "^2.0.0",
-        "polyserve": "^0.23.0",
+        "polyserve": "^0.27.13",
         "puppeteer": "^1.6.1"
     }
 }