Add podman package to travis CI

- https://podman.io/getting-started/installation.html

Author: praveenkumar

.travis.yml

@@ -5,6 +5,12 @@ language: go
 go:
 - 1.13.4
 
+before_script:
+  - echo "deb https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_18.04 /" | sudo tee /etc/apt/sources.list.d/devel:kubic:libcontainers:stable.list
+  - curl -L https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_18.04/Release.key | sudo apt-key add -
+  - sudo apt-get update -qq
+  - sudo apt-get -qq -y install podman slirp4netns
+
 script:
 - make
 - make vendorcheck