CRIU tests are failing on CI

```
=== RUN   TestUsernsCheckpoint
--- FAIL: TestUsernsCheckpoint (0.57s)
    checkpoint_test.go:260: Did not restore the pipe correctly: 
=== RUN   TestCheckpoint
--- FAIL: TestCheckpoint (0.49s)
    checkpoint_test.go:260: Did not restore the pipe correctly: 
```

Seems Travis recently updated the kernel to an incompatible version: https://github.com/containerd/containerd/pull/3898