Update HTML fixtures.
WIP: Replace unmaintained syntax gem with coderay for optional syntax
highlighting.
Tidy up converter interface.

This changes the API but only for private classes.
Better wording on the gem install recommendation.
Regenerate MRI html fixtures
Add changelog entry for syntax -> coderay
Regenerate the rbx html fixtures
Specify looser constraint for coderay version

* Tweak style of coderay warning string
Merge pull request #943 from rspec/issue-932

Replace syntax gem with coderay (now with fixtures)