Investigate TestWaitForObjectMissing timing issue

see https://travis-ci.org/deis/builder/builds/113820760:

``` console
--- FAIL: TestWaitForObjectMissing (0.01s)
    assert.go:40: /go/src/github.com/deis/builder/pkg/gitreceive/storage/object_test.go:92: the statter wasn't called at least once
```
