Add '-loader' suffix to webpack config (#722)

When copy / pasting the webpack configuration given in the 'Testing' doc page, I end up with following error:
ERROR in Entry module not found: Error: Can't resolve 'babel' in '<...>'
BREAKING CHANGE: It's no longer allowed to omit the '-loader' suffix when using loaders. You need to specify 'babel-loader' instead of 'babel'.
My version of webpack: 2.2.0-rc.3
Adding the '-loader' suffix fixed the problem.
Not sure though how safe it is to use 'babel-loader' instead of 'babel' with previous webpack versions...

docs/en/testing.md

@@ -189,7 +189,7 @@ module.exports = {
     loaders: [
       {
         test: /\.js$/,
-        loader: 'babel',
+        loader: 'babel-loader',
         exclude: /node_modules/
       }
     ]