Incompatibility with eslint v4

It seems like 6a0111dd259b2a8eb396a10682144e855bd90d69, which upgrades the dependency to version 4 of eslint, broke the build. From what I see, the problem is [this](http://eslint.org/docs/user-guide/migrating-to-4.0.0#-ruletester-now-validates-properties-of-test-cases).

I'm not familiar enough with the codebase to propose a solution, except "downgrade to v3". It seems like custom properties in test cases are used a lot, so fixing it it's not trivial, but let me know if I can help in some way.