Refactor away babel-core dependency

[kentcdodds]

Due to kentcdodds/babel-plugin-macros#41 we want to refactor babel-core away from our dependencies:

1. Remove babel-core from the package.json dependencies.
2. Remove the require here and instead accept babel as a item in the options argument.
3. Change this to babel and move the destructuring to a const on the next line.
4. Pass babel to each use of getReplacement

I think that's it. Anyone wanna do that?

[xjlim]

I'd like to work on this.

[kentcdodds]

Awesome. Thanks @xjlim!