Add eslint 4 to peer dependencies

[jrzimmerman]

This updates the peer dependencies to accept either eslint version 3 or 4, to remove warning on install.

eslint-plugin-react does this as shown here:
https://github.com/yannickcr/eslint-plugin-react/blob/master/package.json#L38

[coveralls]

Coverage remained the same at 71.91% when pulling cdf4daa on jrzimmerman:master into e78570f on Intellicode:master.

[Intellicode]

Thanks!