Update lib/heroku_san/stage.rb

A tag can be used as the sha, and a commit can be used if the ref is peeled back into
a commit using '^{commit}'

also:
"d9e226c65fc04aea9c7fe1adce771b480688c206\^{commit}\^{commit}" 
  == d9e226c65fc04aea9c7fe1adce771b480688c206