Added support for NextVersion.txt in GitFlow and dynamic repositories

Note that this only applies to develop-based branches. Master branches are still retrieved correctly by using the tags, but this allows to override the next version on develop builds (so I can deploy 4.0.0-unstable0389 on the develop branch, otherwise it would have been 3.10.0-unstable0389).

![image](https://cloud.githubusercontent.com/assets/1246444/3509058/9a4ec1b0-0695-11e4-8771-e1f431f0a4f6.png)