Maintain Ruby 1.8.7 compat

Regexp::NOENCODING does not exist on Ruby 1.8.7:

      it looks like we *could* pass in the literal value of that constant
      into Regexp.new for 1.8.7 and hope that works right, but I don't trust
      that, especially since the Regex engine was changed between 1.8.7 and
      1.9.x.

      This should be good enough to get tests passing on 1.8.7.