Update to React 17 RC2 (#3276)

Author: gaearon

package.json

@@ -18,7 +18,7 @@
     "eslint-plugin-react": "^7.4.0",
     "eslint-plugin-relay": "^0.0.19",
     "flow-bin": "^0.56.0",
-    "gatsby": "^2.0.0",
+    "gatsby": "^2.24.63",
     "gatsby-plugin-catch-links": "^2.0.0",
     "gatsby-plugin-feed": "^2.0.0",
     "gatsby-plugin-glamor": "^2.0.0",
@@ -47,8 +47,8 @@
     "normalize.css": "^8.0.0",
     "prettier": "^1.7.4",
     "prismjs": "^1.15.0",
-    "react": "^16.13.1",
-    "react-dom": "^16.13.1",
+    "react": "^17.0.0-rc.2",
+    "react-dom": "^17.0.0-rc.2",
     "react-helmet": "^5.2.0",
     "react-live": "1.8.0-0",
     "remarkable": "^1.7.1",