chore: Upgrade less

We used to avoid less v3.12 because of a bug, but it seems to be fixed
in less v3.13.

Change-Id: I83a517cec264d4f00af5e1ff31e9be68d5cef2e1

package.json

@@ -47,7 +47,7 @@
     "karma-sourcemap-loader": "^0.3.7",
     "karma-spec-reporter": "0.0.32",
     "karma-webdriver-launcher": "^1.0.8",
-    "less": "^3.9.0 <3.12.0",
+    "less": "^3.13.1",
     "less-plugin-clean-css": "^1.5.1",
     "material-design-lite": "^1.3.0",
     "mux.js": "^5.7.0",