Cleanup package.json

[eps1lon]

• remove unused dependencies
• fixes repository link

Stumbled upon this while looking at dependents of babel-plugin-tester in npm and the repository link for this package gave a 404 and babel-plugin-tester wasn't even used unfortunately.

[dmpinheiro]

Any updates on this? babel-plugin-tester is forcing to download a deprecated version of core-js (version 2), which affects the building process when used. The core-js@2 needs to build bindings from C libraries when installed.
Would be excellent to remove this dependency and generate a new updated version of it.

[kentcdodds]

What does this package have to do with babel-plugin-tester?

[kentcdodds]

Ah, just noticed this PR removes it. That makes sense. Unfortunately I'm not a maintainer on this package so I'm afraid I probably shouldn't do anything here.

/me backs away slowly