Switch to xenial build environment

.travis.yml

@@ -3,14 +3,12 @@ language: c++
 git:
   depth: 1
 
-dist: trusty
+dist: xenial
 
 sudo: true
 
 install:
- - echo "deb http://de.archive.ubuntu.com/ubuntu xenial main restricted universe multiverse" | sudo tee -a /etc/apt/sources.list
- - sudo apt-get update
- - sudo apt-get install --allow-unauthenticated g++ libboost-all-dev cmake libreadline-dev libssl-dev autoconf parallel
+ - sudo apt-get install --allow-unauthenticated g++ libboost-all-dev cmake libssl-dev autoconf parallel
 
 addons:
   sonarcloud: