ENH: Increase coverage for defuzzify submodule; remove `vertex` function

`vertex` used `eval` which is dangerous and has compatibility issues
between Python major versions 2 and 3. Until this can be resolved
(potentially with sympy) and user(s) request it, the `vertex`
function is hereby removed.
Merge pull request #25 from JDWarner/cover_defuzz

ENH: Increase coverage for defuzzify submodule; remove `vertex` function