ci: switch to Ubuntu 18.04 in Travis

Currently, the Travis CI is using Ubuntu 16.04 LTS (Xenial) which is
becoming increasingly outdated. This patch updates Travis to use Ubuntu
18.04 LTS (Bionic) which will give us the benefit of more up-to-date
packages being available and the newer features that come with them.

Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Andrzej Ostruszka <amo@semihalf.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>

.travis.yml

@@ -4,7 +4,7 @@ compiler:
   - gcc
   - clang
 
-dist: xenial
+dist: bionic
 
 os:
   - linux
@@ -118,7 +118,6 @@ matrix:
   - env: DEF_LIB="static"
     arch: arm64
     compiler: gcc
-    dist: bionic
     addons:
       apt:
         packages:
@@ -126,14 +125,12 @@ matrix:
   - env: DEF_LIB="shared"
     arch: arm64
     compiler: gcc
-    dist: bionic
     addons:
       apt:
         packages:
           - *required_packages
   - env: DEF_LIB="static"
     arch: arm64
-    dist: bionic
     compiler: clang
     addons:
       apt:
@@ -141,7 +138,6 @@ matrix:
           - *required_packages
   - env: DEF_LIB="shared"
     arch: arm64
-    dist: bionic
     compiler: clang
     addons:
       apt:
@@ -150,7 +146,6 @@ matrix:
   - env: DEF_LIB="shared" OPTS="-Denable_kmods=false" BUILD_DOCS=1
     arch: arm64
     compiler: gcc
-    dist: bionic
     addons:
       apt:
         packages: