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

[thomasd-]

When using this resource with a webhook trigger, the resource sometimes fails to detect new pull request because Bitbucket calls the webhook URL before creating the refs in the git repository.

Since the resource used git ls-remote to check for new PRs, it happens often that the repo is not ready when the check is done, and then no new version is detected and no job is started.

This commit change the behavior of the check command to use an API call to get the pull requests, which will hopefully be up to date when the webhook calls for the update.

Since we don't have access to the pull-request/<id>/merge hash from the API, the structure of the version is slightly changed :

• hash is replace by from_hash and to_hash, pointing to the HEAD of the source and target branches
• to_hash is only populated when the source indicates that a new build is to be made when the target change. This ensure that a new version is made only when needed.

[mmb]

I'm sorry this pull request has been open for this long. I'll review it and merge it. Is there anything you would change about it since so much time has gone by?