downgrade gatsby to 2.10.5

Gatsby@2.11.0 Build failure
gatsbyjs/gatsby#15190 (comment)
> gatsby-plugin-netlify-cms runs an independent Webpack build within
> Gatsby's Webpack build (insert "yo dawg" reference here) using a
> modified version of the Gatsby Webpack config. Gatsby 2.11.0 no longer
> excludes node_modules, which causes the plugin's Webpack build to attempt
> processing the Netlify CMS modules with Babel. The modules are massive
> and also prebuilt, so it will (and should) crash your system.

> The PR I raised

fix(gatsby-plugin-netlify-cms): exclude node_modules from cms builds
gatsbyjs/gatsby#15191

> just adds that exclusion back in to the plugin's Webpack config.
> Please don't override your node max file size limit to fix this!

feat(gatsby): enable babel-loader for all dependencies
gatsbyjs/gatsby#14111

package.json

@@ -7,7 +7,7 @@
   "private": true,
   "dependencies": {
     "bulma": "0.7.5",
-    "gatsby": "2.11.3",
+    "gatsby": "2.10.5",
     "gatsby-image": "2.2.3",
     "gatsby-plugin-lodash": "3.1.0",
     "gatsby-plugin-manifest": "2.2.0",