Travis CI: dist: xenial is now the default and sudo is deprecated

The default Travis distro is now xenial and the __sudo:__ tags is now deprecated because the sudo is _always_ available and there is no way to turn it off.

Signed-off-by: Alexei Barantsev <barancev@gmail.com>

.travis.yml

@@ -1,8 +1,12 @@
-dist: trusty
-sudo: required
+dist: xenial
+services:
+  - xvfb
+addons:
+  apt:
+    packages:
+      - oracle-java8-installer
 env:
   global:
-    - DISPLAY=:99.0
     - PYTEST_ADDOPTS="--verbose --instafail"
     - FF_ESR_BINARY=$HOME/firefox-52.9.0esr/firefox/firefox-bin
     - FF_NIGHTLY_BINARY=$HOME/firefox-latest-nightly/firefox/firefox-bin

scripts/travis/before_script.sh

@@ -1,6 +1,6 @@
 set -ex
 
-sh /etc/init.d/xvfb start
+# sh /etc/init.d/xvfb start
 
 if [[ ! -z $TOXENV ]]; then	
   ./go py_prep_for_install_release