Switch to eval-source-map (facebook#5060)

Switches to eval-source-map, which is easier for webpack to recompute
when a file changes and has column based mappings, which lets firefox
devtools show original variable in the debugger and evaluate original
expressions in the console.

packages/react-scripts/config/webpack.config.js

@@ -131,7 +131,7 @@ module.exports = function(webpackEnv) {
       ? shouldUseSourceMap
         ? 'source-map'
         : false
-      : isEnvDevelopment && 'cheap-module-source-map',
+      : isEnvDevelopment && 'eval-source-map',
     // These are the "entry points" to our application.
     // This means they will be the "root" imports that are included in JS bundle.
     entry: [