Version bump postcss-preset-env to latest (facebook#5721)

Author: BPScott

package.json

@@ -58,7 +58,7 @@
     "pnp-webpack-plugin": "1.1.0",
     "postcss-flexbugs-fixes": "4.1.0",
     "postcss-loader": "3.0.0",
-    "postcss-preset-env": "6.0.6",
+    "postcss-preset-env": "6.3.1",
     "postcss-safe-parser": "4.0.1",
     "react-app-polyfill": "^0.1.3",
     "react-dev-utils": "^6.1.1",