Add missing import of 'invariant' function

[damiansk]

Hi, I'm using your package in one of my projects. After dependencies update, I notice missing import in your code which is causing error in my production build. 'invariant' package is listed in your package.json so I think that is simple accidental mistake. Please take a look at that single line update and left your thoughts about it, thanks :)

[pachewise]

Fixes #60