Use cliver gem for detecting python 2 binary

I have python v3 installed on my path before python v2 and don't have a python2 binary, so I was getting weird behavior and failing tests (`MentosError: Failed to get header.` a la #45)

A library I wrote for command-line dependency version detection called `cliver` seems to be a good solution here. If python v 2.x cannot be found, it raises an exception that informs the user of the failed dependency in a sensible manner. Cliver avoids shelling out to `which`, and is therefore also more cross-compatible.