travis: install Open vSwitch and create ovsbr0 for integration test

Author: mdlayher

.travis.yml

@@ -5,6 +5,8 @@ os:
   - linux
 sudo: required
 before_install:
+  - sudo apt install openvswitch-switch
+  - sudo ovs-vsctl add-br ovsbr0
   - go get github.com/golang/lint/golint
   - go get honnef.co/go/tools/cmd/staticcheck
   - go get -d ./...