Update README badges

README badges are fairly common, and often refer to the branch name in their URLs:

```
[![Build status](https://travis-ci.org/org/repo.svg?branch=master)](https://travis-ci.org/org/repo)
```

We could probably figure out some patterns like <code>org/repo.svg?branch=old_branch_name</code> which can be replaced with <code>org/repo.svg?branch=new_branch_name</code>.