ci: sudo microk8s.$command

Apparently microk8s commands now require `sudo` to run [1]. Even
`kubectl`. It's a bit annoying to have to do this, but without it all of
our builds are blocked.

According to microk8s itself, we could try a different approach:

```
Insufficient permissions to access MicroK8s.
You can either try again with sudo or add the user travis to the 'microk8s' group:
    sudo usermod -a -G microk8s travis
The new group will be available on the user's next login.
```

Although that would require a new login during a CI run, which I'm also
not a fan of.

[1] canonical/microk8s#640

.travis.yml

@@ -7,13 +7,14 @@ services:
 - docker
 before_install:
 - sudo snap install microk8s --classic --channel=1.14/stable
-- sudo snap alias microk8s.kubectl kubectl
+- curl -LO https://storage.googleapis.com/kubernetes-release/release/v1.14.6/bin/linux/amd64/kubectl > $(go env GOPATH)/bin/kubectl
+- chmod +x $(go env GOPATH)/bin/kubectl
 - mkdir ~/.kube
-- microk8s.config > ~/.kube/config
+- sudo microk8s.config > ~/.kube/config
 - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh |
   sh -s -- -b $(go env GOPATH)/bin v1.17.1
-- microk8s.status --wait-ready
-- microk8s.enable registry
+- sudo microk8s.status --wait-ready
+- sudo microk8s.enable registry
 install: true
 script:
 - "./ci/test.sh && ./ci/build.sh && ./ci/e2e.sh"