Ensure dependencies are correctly declared

[kadamwhite]

As noted in #414, v1.2.0 was released without properly identifying that li was listed as a devDependency instead of a regular dependency. We should either implement an eslint rule to validate imports, or else add a build validation step to test out a dependencies-only install before releasing, to spare the developers who depend on this library some frustration.

[KenEucker]

+1 to this

I am trying to get this library to work but I get the following error when including the package:

Error: Cannot find module 'li'
Require stack:
- /XXX/node_modules/wpapi/lib/autodiscovery.js
- /XXX/node_modules/wpapi/wpapi.js