(MAINT) add redcarpet gem for travis errors

- add redcarpet gem (pinned to 1.17.2) required by
  yard

- do not run yard on ruby 1.8 because of this:

  [error]: Missing 'redcarpet' gem for Markdown formatting. Install it
  with `gem install redcarpet`
Merge pull request #231 from anodelman/1.8-travis-failures

(MAINT) add redcarpet gem for travis errors