Update Travis CI build

- Migrate to Xenial
 - Use whitelisted PPA instead of hardcoded deb URL
 - Only add X11 dependencies for X11 builds
 - Only set sudo on matrix entries

.travis.yml

@@ -4,9 +4,12 @@ branches:
     only:
         - ci
         - master
-sudo: false
-dist: trusty
-addons:
+matrix:
+    include:
+        - os: linux
+          dist: xenial
+          sudo: false
+          addons:
               apt:
                   packages:
                       - cmake
@@ -14,21 +17,32 @@ addons:
                       - libxinerama-dev
                       - libxcursor-dev
                       - libxi-dev
-matrix:
-    include:
-        - os: linux
           env:
               - BUILD_SHARED_LIBS=ON
               - CFLAGS=-Werror
         - os: linux
+          dist: xenial
+          sudo: false
+          addons:
+              apt:
+                  packages:
+                      - cmake
+                      - libxrandr-dev
+                      - libxinerama-dev
+                      - libxcursor-dev
+                      - libxi-dev
           env:
               - BUILD_SHARED_LIBS=OFF
               - CFLAGS=-Werror
         - os: linux
+          dist: xenial
           sudo: required
           addons:
               apt:
+                  sources:
+                      - ppa:kubuntu-ppa/backports
                   packages:
+                      - extra-cmake-modules
                       - libwayland-dev
                       - libxkbcommon-dev
                       - libegl1-mesa-dev
@@ -37,10 +51,14 @@ matrix:
               - BUILD_SHARED_LIBS=ON
               - CFLAGS=-Werror
         - os: linux
+          dist: xenial
           sudo: required
           addons:
               apt:
+                  sources:
+                      - ppa:kubuntu-ppa/backports
                   packages:
+                      - extra-cmake-modules
                       - libwayland-dev
                       - libxkbcommon-dev
                       - libegl1-mesa-dev
@@ -49,10 +67,12 @@ matrix:
               - BUILD_SHARED_LIBS=OFF
               - CFLAGS=-Werror
         - os: osx
+          sudo: false
           env:
               - BUILD_SHARED_LIBS=ON
               - CFLAGS=-Werror
         - os: osx
+          sudo: false
           env:
               - BUILD_SHARED_LIBS=OFF
               - CFLAGS=-Werror
@@ -60,9 +80,7 @@ script:
     - if grep -Inr '\s$' src include docs tests examples CMake *.md .gitattributes .gitignore; then echo Trailing whitespace found, aborting.; exit 1; fi
     - mkdir build
     - cd build
-    - if test -n "${USE_WAYLAND}";
-          then wget https://mirrors.kernel.org/ubuntu/pool/universe/e/extra-cmake-modules/extra-cmake-modules_5.38.0a-0ubuntu1_amd64.deb;
-          sudo dpkg -i extra-cmake-modules_5.38.0a-0ubuntu1_amd64.deb;
+    - if test -n "${USE_WAYLAND}"; then
           git clone git://anongit.freedesktop.org/wayland/wayland-protocols;
           pushd wayland-protocols;
           git checkout 1.15 && ./autogen.sh --prefix=/usr && make && sudo make install;