Fix missing "React Helmet" dependency

During the initial implementation of Gatsby (GH-27) the
gatsby-plugin-react-helmet (1) has been added, but the required
react-helmet (2) main package was accidentally forgotten to be added.

References:
  (1) https://www.npmjs.com/package/gatsby-plugin-react-helmet
  (2)https://www.npmjs.com/package/react-helmet

Related to GH-27
Closes GH-34

Author: arcticicestudio

package-lock.json

@@ -75,6 +75,7 @@
     "gatsby-transformer-yaml": "2.1.5",
     "nord": ">=0.2.1 <1.0.0",
     "react": "16.6.3",
-    "react-dom": "16.6.3"
+    "react-dom": "16.6.3",
+    "react-helmet": "5.2.0"
   }
 }