Split @babel/runtime into 2 modules via @babel/runtime-corejs2

[nicolo-ribaudo]

babel/babel#8266

Fixes #70

[hzoo]

Should we remove the original runtime? Don't need both. But it depends on if they want corejs or not. Technically if they used helpers true or false, or polyfill true/false it would be different as well but it gets complicated haha

And what happens if you use transform-runtime with options already?

[nicolo-ribaudo]

I updatet the PR:

• Don't duplicate the runtime in the dependencies
• Replace runime -> runtime-corejs only when the old version did already inclue the runtime:

  @babel/runtime: 7.0.0-beta.35 -> @babel/runtime-corejs
  @babel/runtime: 7.0.0-rc.1 -> @babel/runtime
  

• Only add the corejs option when upgrading from Babel 6. Display a warning for Babel 7 since it could be an old beta with corejs included in babel/runtime.

[existentialism]

Nit:

Babel was not able to decide whether or not to add a