Allow stage 3 css transforms (facebook#5091)

* Switch to stage 3 css

* Tweak default browsers, too

Author: Timer

config/webpack.config.dev.js

@@ -60,7 +60,7 @@ const getStyleLoaders = (cssOptions, preProcessor) => {
             autoprefixer: {
               flexbox: 'no-2009',
             },
-            stage: 4,
+            stage: 3,
           }),
         ],
       },

config/webpack.config.prod.js

@@ -72,7 +72,7 @@ const getStyleLoaders = (cssOptions, preProcessor) => {
             autoprefixer: {
               flexbox: 'no-2009',
             },
-            stage: 4,
+            stage: 3,
           }),
         ],
         sourceMap: shouldUseSourceMap,