MINOR: Update Travis build environment to Ubuntu 20.04 Focal Fossa

Author: dkorunic

.travis.yml

@@ -2,6 +2,7 @@ language: go
 go:
 - 1.x
 sudo: false
+dist: focal
 before_script:
   - sudo curl -sfSL https://github.com/haproxytech/dataplaneapi/releases/download/v2.1.0/dataplaneapi_2.1.0_Linux_x86_64.tar.gz -o dataplane.tar.gz
   - sudo mkdir /tmp/dataplane