Avoid slow operations in FormulaPin#initialize

A FormulaPin object is created every time Formula is instantiated, so
don't do filesystem operations or Pathname concatenation eagerly.
Performance fix for Pathname#chop_basename

This is an internal method, but is called a bunch of times in
performance-critical codepaths, and is ultra slow because the constant
is interpoplated into the Regexp each time the method is called.

Alas, this has been fixed in Ruby 1.9+.