Remove babel-polyfill
react-sdk's tests are failing because babel-polyfill is being
pulled in twice.
As per https://babeljs.io/docs/usage/polyfill/ babel-polyfill is
"intended to be used in an application rather than a library/tool".
From a library, we're limited to things that don't modify globals,
since the js env is not ours to start screwing around with.
