Add history as top-level dependency

was causing the webpack build to fail (cf similar problem fixed in gatsbyjs/gatsby#174)

Author: mef

package.json

@@ -13,6 +13,7 @@
   "author": "Kyle Mathews <[email protected]>",
   "license": "MIT",
   "dependencies": {
+    "history": "^2.0.0",
     "front-matter": "^2.0.6",
     "highlight.js": "^9.2.0",
     "markdown-it": "^6.0.0",