Lock some dependencies to avoid failures on Travis
Lock some dependencies to avoid failures on Travis

Nokogiri 1.6 doesn't support Ruby 1.8.7 anymore, we should lock it
to 1.5.9. Lock other development dependencies to prevent this kind
of issues in the future.

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