enh: use ubuntu 20.04 as build image on travis

Author: sanjayankur31

.travis.yml

@@ -2,17 +2,14 @@ language: cpp
 
 sudo: required
 
-dist: xenial
+dist: focal
 
 compiler:
     - gcc
 
 install:
     - sudo apt-get update -qq
-    - sudo apt-get install apt-transport-https ca-certificates gnupg software-properties-common wget
-    - wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | sudo tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/null
-    - sudo apt-add-repository 'deb https://apt.kitware.com/ubuntu/ xenial main'
-    - sudo apt-get install -y -qq cmake libxerces-c-dev xsdcxx
+    - sudo apt-get install wget libxerces-c-dev xsdcxx cmake -qq
 
 script:
     - ls -alt