Build without jobs

Creating `Build` objects using `t.builds(..)` means they do not have a `jobs` attribute

```
t = travispy.TravisPy()

builds = t.builds(slug='menegazzo/travispy')

assert len(builds) > 1
assert all(isinstance(build, travispy.entities.build.Build)
           for build in builds)

build = builds[0]
assert hasattr(build, 'jobs')
```
