Use babel-loader 7 beta. (#1495)

* Use babel-loader 7 beta.
This will get rid of the DeprecationWarning printed while
starting next.
Also this is the version recommened to use with Webpack 2.

* Update to the babel-loader-7

package.json

@@ -46,7 +46,7 @@
     "ansi-html": "0.0.7",
     "babel-core": "6.24.0",
     "babel-generator": "6.24.0",
-    "babel-loader": "6.4.1",
+    "babel-loader": "7.0.0",
     "babel-plugin-module-resolver": "2.6.2",
     "babel-plugin-react-require": "3.0.0",
     "babel-plugin-transform-class-properties": "6.22.0",

server/build/webpack.js

@@ -146,7 +146,6 @@ export default async function createCompiler (dir, { dev = false, quiet = false,
 
   const mainBabelOptions = {
     cacheDirectory: true,
-    sourceMaps: dev ? 'both' : false,
     presets: []
   }
 
@@ -246,7 +245,6 @@ export default async function createCompiler (dir, { dev = false, quiet = false,
     options: {
       babelrc: false,
       cacheDirectory: true,
-      sourceMaps: dev ? 'both' : false,
       presets: [require.resolve('./babel/preset')]
     }
   }, {