Update dependency gatsby-mdx to v0.3.4

Author: renovate-bot

package.json

@@ -43,7 +43,7 @@
     "countries-list": "2.4.1",
     "documentation": "9.1.1",
     "gatsby": "2.0.91",
-    "gatsby-mdx": "0.1.4",
+    "gatsby-mdx": "0.3.4",
     "gatsby-plugin-google-analytics": "2.0.9",
     "gatsby-plugin-manifest": "2.0.13",
     "gatsby-plugin-offline": "2.0.21",