Attempt to make PrismJS work

Testing this:
TryGhost/gatsby-starter-ghost#25 (comment)

.babelrc

@@ -0,0 +1,28 @@
+{
+    "presets": [
+      [
+        "babel-preset-gatsby"
+      ]
+    ],
+    "plugins": [
+      [
+        "prismjs",
+        {
+          "languages": [
+            "javascript",
+            "css",
+            "markup",
+            "json",
+            "handlebars",
+            "yaml",
+            "java",
+            "sql",
+            "dart",
+            "xml"
+          ],
+          "theme": "okaidia",
+          "css": true
+        }
+      ]
+    ]
+  }

package.json

@@ -39,6 +39,8 @@
   "dependencies": {
     "@tryghost/helpers": "1.1.19",
     "@tryghost/helpers-gatsby": "1.0.22",
+    "babel-plugin-prismjs": "^1.1.1",
+    "babel-preset-gatsby": "^0.2.26",
     "cheerio": "1.0.0-rc.3",
     "gatsby": "2.17.15",
     "gatsby-awesome-pagination": "0.3.5",
@@ -55,6 +57,7 @@
     "gatsby-source-ghost": "4.0.3",
     "gatsby-transformer-sharp": "2.3.5",
     "lodash": "4.17.15",
+    "prismjs": "^1.17.1",
     "react": "16.12.0",
     "react-dom": "16.12.0",
     "react-helmet": "5.2.1"