Remove duplicate transform-regenerator dependency

[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.

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