Auto-branch updater for open pull requets

If pull requests are open and need to be updated with base branch, bot can basically update the branch in case of no merge conflict.

```
https://developer.github.com/v3/repos/merging/
```

```
{
 "base": "your_branch",
 "head": "master",
 "commit_message": "Merge master into branch"
}
```