Refactor the way commands are required

Fix issue #834 (pry-0.9.11.4 failing on Windows 7 64 bit - cannot load
such file)

The problem was introduced by this commit:
25cbe4313729d53cd8254dca8e3ce0022ab499eb

This bug affects *only* Ruby 1.9.3p327, on Windows.

            Choose your destiny
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  TL;DR:                http://bugs.ruby-lang.org/issues/7374
  File::expand_path
  is bugged on
  Ruby 1.9.3p327

This fix is simply an ugly workaround.