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-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