Deployments fail where a branch has \W in the git branch name

Where an r10k git repo uses branches where the name is a version, e.g. 1.2.3 a deployment will fail in `task/deployment.rb` because the `env.dirname` has been sanitised but the branchname which is later used to match is not.  This patch is the simple case of following the gsub performed elsewhere but in a private method.