Upgrade to latest alpha

I'm going to add some new pages soon and I want to dogfood the new
Concurrent + Suspense + Hooks features.

package.json

@@ -45,8 +45,8 @@
     "normalize.css": "^8.0.0",
     "prettier": "^1.7.4",
     "prismjs": "^1.15.0",
-    "react": "^16.4.1",
-    "react-dom": "^16.4.1",
+    "react": "16.7.0-alpha.0",
+    "react-dom": "16.7.0-alpha.0",
     "react-helmet": "^5.2.0",
     "react-live": "1.8.0-0",
     "remarkable": "^1.7.1",