Flake: docker hub requiring authentication

Suddenly having a rash of failures on Travis trying to pull images from Docker Hub. Assuming they're transient.

```
--- FAIL: TestRun (679.14s)
/integration/TestRun/buildpacks_Go
time="2021-02-03T17:49:24Z" level=info msg="Namespace: skaffoldk44vr"
time="2021-02-03T17:49:24Z" level=info msg="Running [skaffold run --namespace skaffoldk44vr --default-repo gcr.io/k8s-skaffold] in examples/buildpacks"
helper.go:229: skaffold run: exit status 1, Generating tags...
- skaffold-buildpacks -> gcr.io/k8s-skaffold/skaffold-buildpacks:c2040d5
Checking cache...
- skaffold-buildpacks: Not found. Building
Found [k3d-k3s-default] context, using local docker daemon.
[...]
v1: Pulling from buildpacks/gcp/run
9f2825a1003a: Already exists
7e79098fe9b4: Already exists
3c2cba919283: Already exists
261de18b35a6: Already exists
c26187d600b1: Already exists
Digest: sha256:578631b17d0db25f29d6157d0390e84e657446f908a26dc1fe70ee161eb8f7b5
Status: Downloaded newer image for gcr.io/buildpacks/gcp/run:v1
0.9.3: Pulling from buildpacksio/lifecycle
panic.go:617: failed to build: fetching lifecycle image: unauthorized: authentication required
--- FAIL: TestRun/buildpacks_Go (26.61s)
/integration/TestRun/kustomize
time="2021-02-03T17:52:58Z" level=info msg="Namespace: skaffoldnrdn7"
time="2021-02-03T17:52:58Z" level=info msg="Running [skaffold run --namespace skaffoldnrdn7 --default-repo gcr.io/k8s-skaffold] in examples/getting-started-kustomize"
helper.go:229: skaffold run: exit status 1, Generating tags...
- skaffold-kustomize -> gcr.io/k8s-skaffold/skaffold-kustomize:c2040d5
Checking cache...
- skaffold-kustomize: Error checking cache.
panic.go:617: failed to build: getting hash for artifact "skaffold-kustomize": getting dependencies for "skaffold-kustomize": parsing ONBUILD instructions: retrieving image "alpine:3.10": GET https://index.docker.io/v2/library/alpine/manifests/3.10: UNAUTHORIZED: authentication required; [map[Action:pull Class: Name:library/alpine Type:repository]]
--- FAIL: TestRun/kustomize (1.21s)
```

and

```
panic.go:617: executing build: getting hash for artifact "skaffold-example": getting dependencies for "skaffold-example": parsing ONBUILD instructions: retrieving image "golang:1.15": GET https://index.docker.io/v2/library/golang/manifests/1.15: UNAUTHORIZED: authentication required; [map[Action:pull Class: Name:library/golang Type:repository]]
```