travis: Make xenial the main distribution.

The end of standard support for trusty ended in April 2019 and
the end of life is April 2022. For xenial the end of standard
support is April 2021 and the end of life is April 2024.

Most users should have xenial or newer at this point.

Source: https://wiki.ubuntu.com/Releases

.travis.yml

@@ -1,6 +1,6 @@
 ---
 cache: ccache
-dist: trusty
+dist: xenial
 language: cpp
 services: xvfb
 sudo: required
@@ -10,10 +10,10 @@ matrix:
     include:
         - env: BUILD_ENV=ubuntu-bionic XVFB_RUN=1
           dist: bionic
-        - env: BUILD_ENV=ubuntu-xenial XVFB_RUN=1
-          dist: xenial
         - env: BUILD_ENV=ubuntu-trusty XVFB_RUN=1
-        - env: BUILD_ENV=ubuntu-trusty XVFB_RUN=1 CC=clang CXX=clang++
+          dist: trusty
+        - env: BUILD_ENV=ubuntu-xenial XVFB_RUN=1
+        - env: BUILD_ENV=ubuntu-xenial XVFB_RUN=1 CC=clang CXX=clang++
         - env: BUILD_ENV=mingw-w64
         - env: BUILD_ENV=mingw-w32
         - env: BUILD_ENV=libretro