travis: upgrade Ubuntu target dist to 16.04 Xenial

Ubuntu 14.04 Trusty is LTS until April 2019 and since we are
approaching that, upgrade to Xenial which is LTS until April 2021.

One bonus of this upgrade is that the Travis Xenial virtual images
have a much bigger disk size, thus no need to limit ccache below
the default of 5G anymore.

Also had to remove linux/clang-8 toolchain testing since
llvm-toolchain-xenial is not listed at:
https://github.com/travis-ci/apt-source-safelist/blob/master/ubuntu.json

And fails with:
Disallowing sources: llvm-toolchain-xenial
...
E: Unable to locate package clang-8
E: Unable to locate package llvm-8-dev

.travis.compiler.sh

@@ -22,7 +22,6 @@ if [[ "${TRAVIS_OS_NAME}" == 'linux' ]]; then
   exclude_modules;
   if which ccache ; then
     CMAKE_OPT="${CMAKE_OPT} -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache"
-    ccache --max-size=2200M
   fi
   if [[ "${CXX}" == 'clang++' ]]; then
     if [[ "${CC_VERSION}" == '6' ]]; then

.travis.yml

@@ -2,7 +2,7 @@
 # travis-ci.org definition
 
 sudo: false
-dist: trusty
+dist: xenial
 
 git:
   depth: 2
@@ -40,6 +40,9 @@ matrix:
     - os: osx
       compiler: clang
       env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
+    - os: linux
+      compiler: clang
+      env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
   include:
     - os: linux
       compiler: gcc
@@ -83,13 +86,13 @@ matrix:
             - uuid-dev
             - devscripts
             - fakeroot
+            - dh-systemd
+            - libsystemd-dev
+            - libzstd-dev
+            - unixodbc-dev
       script:
         - ${CC} --version ; ${CXX} --version
-      # https://github.com/travis-ci/travis-ci/issues/7062 - /run/shm isn't writable or executable
-      # in trusty containers
-        - export MTR_MEM=/tmp
         - env DEB_BUILD_OPTIONS="parallel=4" debian/autobake-deb.sh;
-        - ccache --show-stats
   # Until OSX becomes a bit more stable: MDEV-12435 MDEV-16213
   allow_failures:
     - os: osx
@@ -110,17 +113,13 @@ matrix:
     - os: osx
       compiler: clang
       env: CC_VERSION=7 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=csv,federated,funcs_1,funcs_2,gcol,handler,heap,json,maria,perfschema,plugins,multi_source,roles
-    - os: linux
-      compiler: clang
-      env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
 
 addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
-      - llvm-toolchain-trusty-6.0
-      - llvm-toolchain-trusty-7
-      - llvm-toolchain-trusty
+      - llvm-toolchain-xenial-6.0
+      - llvm-toolchain-xenial-7
     packages: # make sure these include all compilers and all build dependencies (see list above)
       - gcc-6
       - g++-6
@@ -132,8 +131,6 @@ addons:
       - llvm-6.0-dev
       - clang-7
       - llvm-7-dev
-      - clang-8
-      - llvm-8-dev
       - bison
       - chrpath
       - cmake
@@ -176,6 +173,7 @@ before_script:
   - df -h
   - ccache --version
   - ccache --show-stats
+  - ccache --max-size=5G
 
 script:
 # following modules are disabled after sourcing .travis.compiler.sh:
@@ -193,6 +191,8 @@ script:
          --suite=${MYSQL_TEST_SUITES}
          --skip-test-list=unstable-tests
          --skip-test=binlog.binlog_unsafe
+
+after_script:
   - ccache --show-stats
   - df -h