Removes pending UIKit spec.

[Project collaborators should not suffer ever-present pending test in UIKit specs](https://www.pivotaltracker.com/story/show/67151604)

Cleans up `heightWithWidth:font:` and spec.
* Removes `if` in specs which removes pending specs.
* Uses `respondsToSelector:` over explicitly checking iOS version.
* Only one path is run per iOS version (CI runs against both).