Upgrade Babel to v7 #392
Description
As suggested in pr #389 (https://github.com/nodejs/readable-stream/pull/389/files#r241132850). Used babel packages are old and can be upgraded.
It is actually not possible to build readable-stream without relying on alpha code ("babel-plugin-transform-optional-catch-binding": "^7.0.0-alpha.16")