remove postcss-flexbugs-fixes from build process

Author: mdo

build/postcss.config.js

@@ -35,7 +35,6 @@ module.exports = (ctx) => ({
         'Android >= 4',
         'Opera >= 12'
       ]
-    },
-    'postcss-flexbugs-fixes': {}
+    }
   }
 })

package.json

@@ -86,7 +86,6 @@
     "npm-run-all": "^4.0.2",
     "phantomjs-prebuilt": "^2.1.14",
     "postcss-cli": "^4.0.0",
-    "postcss-flexbugs-fixes": "^3.0.0",
     "qunit-phantomjs-runner": "^2.3.0",
     "qunitjs": "^2.3.2",
     "shelljs": "^0.7.7",