Pin redcarpet dependency to make Ruby 1.8.7 happy

Travis tests for Ruby 1.8.7 have been failing because Redcarpet 3.x dropped
support for 1.8.x. Pin Redcarpet to 2.3.x to work around this.
Merge pull request #9 from Sharpie/fix-1.8.7

Pin redcarpet dependency to make Ruby 1.8.7 happy