Pin a version of grunt-wordpress to 2.1.4

`grunt-wordpress` updates `gilded-wordpress` in patch releases. However,
our infra requires strict matching of the `gilded-wordpress` version
in the `GW_VERSION` variable set in `gilded-wordpress.php`. Let's pin
the version to avoid breaking changes from subdependencies updates.

package.json

@@ -23,7 +23,7 @@
     "async": "^3.2.0",
     "cheerio": "^1.0.0-rc.12",
     "grunt-check-modules": "^1.1.0",
-    "grunt-wordpress": "^2.1.4",
+    "grunt-wordpress": "2.1.4",
     "he": "^1.2.0",
     "highlight.js": "^10.7.2",
     "marked": "^4.0.0",