Log Message:
Update ruby-sexp-processor to 4.2.1.

=== 4.2.1 / 2013-04-09

* 1 bug fix:

  * Removed structure_unused_literal_wwtt because I just don't care anymore

=== 4.2.0 / 2013-03-18

* 2 minor enhancements:

  * Added SexpInterpreter since it is such a common pattern
  * Added aliases Sexp#head & Sexp#rest for readability