Run CI against ruby-head
Run CI against ruby-head

To get a better feedback on errors and be more or less ready for 2.1 for
the next release of Redcarpet.

emq/redcarpet,JuanitoFatas/redcarpet,be9/redcarpet,emq/redcarpet,Hacker0x01/redcarpet,increments/greenmat,fukayatsu/redcarpet,JuanitoFatas/redcarpet,liquorburn/redcarpet,kattybilly/redcarpet,gitcafe-dev/redcarpet,kaneshin/redcarpet,emq/redcarpet,fukayatsu/redcarpet,liquorburn/redcarpet,be9/redcarpet,be9/redcarpet,kaneshin/redcarpet,increments/greenmat,JuanitoFatas/redcarpet,emq/redcarpet,Hacker0x01/redcarpet,gitcafe-dev/redcarpet,kattybilly/redcarpet,gitcafe-dev/redcarpet,vmg/redcarpet,increments/greenmat,vmg/redcarpet,Hacker0x01/redcarpet,fukayatsu/redcarpet,fukayatsu/redcarpet,kattybilly/redcarpet,kattybilly/redcarpet,be9/redcarpet,liquorburn/redcarpet,Hacker0x01/redcarpet,liquorburn/redcarpet,vmg/redcarpet,vmg/redcarpet,kaneshin/redcarpet,kaneshin/redcarpet,increments/greenmat