Restrict activesupport dependency to pre-4.0.0 to maintain <= 1.9.2 compatibility.
Default Markdown template parser for new sites to RedCarpet via _config.yml. Related: #1236.
Update history to reflect restriction of activesupport dependency.
Merge pull request #1245 from mojombo/jekyll-new-redcarpet

Default Markdown template parser for new sites to RedCarpet
Update history to reflect merge of #1245
Make redcarpet a runtime dependency
Update the QuickStart guide for the default engine change

As of Jekyll 1.1, we'll be using Redcarpet as the default for all new
sites.
Merge pull request #1247 from mojombo/jekyll-new-redcarpet

Complete the transition to Redcarpet for new sites
Update history to reflect merge of #1247.