travis: bump the Ubuntu release to Bionic

The main reason why this is necessary is that gcc 5.x on Xenial doesn't
support ASan on s390x. Bumping the release to Bionic with gcc 7.x allows
us to build libbpf on s390x with ASan without issues.

.travis.yml

@@ -1,5 +1,5 @@
 sudo: required
-dist: xenial
+dist: bionic
 services:
     - docker
 
@@ -124,27 +124,27 @@ jobs:
           after_script:
               - $CI_MANAGERS/debian.sh CLEANUP
 
-        - name: Ubuntu Xenial
+        - name: Ubuntu Bionic
           language: bash
           script:
               - set -e
-              - sudo $CI_MANAGERS/xenial.sh
+              - sudo $CI_MANAGERS/ubuntu.sh
               - set +e
 
-        - name: Ubuntu Xenial (arm)
+        - name: Ubuntu Bionic (arm)
           arch: arm64
           language: bash
           script:
               - set -e
-              - sudo $CI_MANAGERS/xenial.sh
+              - sudo $CI_MANAGERS/ubuntu.sh
               - set +e
 
-        - name: Ubuntu Xenial (s390x)
+        - name: Ubuntu Bionic (s390x)
           arch: s390x
           language: bash
           script:
               - set -e
-              - sudo $CI_MANAGERS/xenial.sh
+              - sudo $CI_MANAGERS/ubuntu.sh
               - set +e
 
         - stage: Coverity
@@ -161,7 +161,7 @@ jobs:
               - COVERITY_SCAN_BUILD_COMMAND_PREPEND="cd src/"
               - COVERITY_SCAN_BUILD_COMMAND="make"
           install:
-              - sudo echo 'deb-src http://archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse' >>/etc/apt/sources.list
+              - sudo echo 'deb-src http://archive.ubuntu.com/ubuntu/ bionic main restricted universe multiverse' >>/etc/apt/sources.list
               - sudo apt-get update
               - sudo apt-get -y build-dep libelf-dev
               - sudo apt-get install -y libelf-dev pkg-config

travis-ci/managers/ubuntu.sh

@@ -2,7 +2,9 @@
 set -e
 set -x
 
-echo 'deb-src http://archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse' >>/etc/apt/sources.list
+RELEASE="bionic"
+
+echo "deb-src http://archive.ubuntu.com/ubuntu/ $RELEASE main restricted universe multiverse" >>/etc/apt/sources.list
 
 apt-get update
 apt-get -y build-dep libelf-dev