Explicit branch tuple in rebar.config

Recent versions of rebar say versions confict when you have:

```
ERROR: Conflicting dependencies for proper: [{"From: ubic_lib",
                                              {".*",
                                               {git,
                                                "https://github.com/manopapad/proper.git",
                                                {branch,"master"}}}},
                                             {"From: erlasticsearch",
                                              {".*",
                                               {git,
                                                "https://github.com/manopapad/proper.git",
                                                "master"}}}]
```