Remove duplicate transform-regenerator dependency

Created by: akofman

It looks like babel-plugin-transform-regenerator is already a dependency of babel-preset-es2015 in a different version :

$npm ls babel-plugin-transform-regenerator 
babel-preset-react-app@0.2.1 
├── babel-plugin-transform-regenerator@6.14.0
└─┬ babel-preset-latest@6.14.0
  └─┬ babel-preset-es2015@6.16.0
    └── babel-plugin-transform-regenerator@6.16.1

This can causes some trouble when using regeneratorRuntime but to ensure the older npm compatibility we have to keep this dependency. So I just updated its version to the same version found from babel-preset-es2015@6.16.0.