Add BABEL_ENV into build scripts in package.json

[birdofpreyru]

https://github.com/topcoder-platform/topcoder-react-starter/blob/564dc5a6e4d0838c4c0724f94dc5e443f5de2794/package.json#L7-L8

BABEL_ENV=production and BABEL_ENV=development should be added in front of the Webpack build command, otherwise proper environments are not selected during the build (since the update to Babel 7).