Use an API call to check for new versions instead of git ls-remote#17

thomasd- wants to merge 1 commit intolaurentverbruggen:masterlaurentverbruggen/concourse-bitbucket-pullrequest-resource:masterfrom

emerald-squad:check-with-apiemerald-squad/concourse-bitbucket-pullrequest-resource:check-with-apiCopy head branch name to clipboard