Ensure we clean ALL Ruby environment vars

Previously `_ORIGINAL_GEM_PATH` was leaking into Ruby subprocesses when
we shellout and execute the various Ruby functions from the Builder DSL.
Revert "Ensure Builder DSL Ruby methods run under the embedded Ruby"

This reverts commit 697560d532d7ebb5795aca010f9029f1801aab87.
Merge pull request #343 from opscode/schisamo/ruby-env-leak-fix

Leaky Ruby ENV fix