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.

Author: jasonLaster

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: [