Revert "Test for whatever `this` values are actually given in the environment"

This reverts commit 67178f3a946d77723892078264c3c881740273c2.

See #15. The change introduced didn't actually work around the PhantomJS-sucks problem, and I'm unwilling to go further into the territory of accomodating-broken-browsers. So let's just revert it back to the simpler version. Full discussion over in that issue.