add support for setting build

I'd like to set the build portion of the version for continuous integration. `node-semver` has [a concept of build](https://github.com/isaacs/node-semver/blob/master/semver.js#L288) but does not expose any way to interact with it. I tried to mimic [their regex](https://github.com/isaacs/node-semver/blob/master/semver.js#L91-L97) to identify the build portion of the version.

Usage:

```bash
grunt release:build:$BUILD_NUMBER
```

`$BUILD_NUMBER` would be an environmental variable set by the CI server or could easily just be a timestamp. 

This is just a first pass and would appreciate feedback :smiley: