Versions of capistrano greater than 2.14.2 use git clone -b BRANCH instead of just git clone, and the -b option of git clone can only take a branch name.
The auto_tagger capistrano recipe sets the branch variable to a git sha, so git clone -b breaks.
That bug was introduced way back in this commit in capistrano capistrano/capistrano@b8ce59c but only released with 2.15.2.
2.14.2 is the last good version, so I've released a new version of auto_tagger that only works with capistrano up to 2.14.2, but this is a hack.
The new version of auto_tagger is 0.2.10, and you have to downgrade capistrano, so this is obviously not a long-term fix.