Log Message:
Update ruby-parslet to 1.8.0.

= 1.8 / 3Apr2017

  + The `ignored` atom that allows to ignore a part of the matched text.
    `str('foo').ignore` will match 'foo', but not yield any parse output.
    Thanks to chrismwendt (Chris Wendt).
  + Infix expression parser (arithmetics, anyone?) now supports custom reducers
    in block form. Thanks to chrismwendt (Chris Wendt).
  ! Small patches to memory footprint (Christophe Bliard).
  - blankslate dependency removed. You should be good - but if things break,
    please let us know (Nikita Shilnikov).
  ! Parslet now has `parse_failure_cause`, replaces the earlier `cause`.
  ! Fixes all these interpreter warnings on modern rubies.