(maint) restrict redcarpet gem to <= 2.3.0

Per the tag annotation from the redcarpet 3.0.0 release, 2.3.0 is the last
version compatible with ruby 1.8.7.

https://github.com/vmg/redcarpet/releases/tag/v3.0.0
Merge pull request #582 from jhoblitt/redcarpet_gem

(maint) restrict redcarpet gem to <= 2.3.0