Build springboot from HEAD

Previously, the `spring boot` formula would only download and install the latest milestone.  For people testing snapshots (and not wanting to do a manual installation), this wasn't entirely helpful.  This change adds support for Homebrew's [unstable version support]().  Users can now do a

```bash
brew install springboot --HEAD
```

and get an install built from the `HEAD` of the Spring Boot repo.

[unstable versions support]: https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook#unstable-versions-head-devel