A firkin is a small cask or a unit of liquid volume equal to half a kilderkin. This word comes from the same time period in the English language when words like 'furlong' and 'fortnight' were used in common parlance.
The spigot from the casket symbolizes the spout out of which tons and tons of code come spilling. Travis is a genius, and a bit on the quirky side. But that IS what makes him a great programmer!