Replace stage & es2017 with specific plugins

Also sort out regenerator runtime as it turns out importing
babel-polyfill in your code is insufficient if using webpack
because it's imported too late, so use both that and
regenerator-runtime. Sigh.

.babelrc

@@ -1,3 +1,4 @@
 {
-  "presets": ["react", "es2015", "es2016", "es2017", "stage-2"]
+  "presets": ["react", "es2015", "es2016"],
+  "plugins": ["transform-class-properties", "transform-object-rest-spread", "transform-async-to-generator", "transform-runtime"]
 }