build: Update Travis-CI config

Container updates for 2017Q2:

https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch
diff --git a/.travis.yml b/.travis.yml
index 1587eed..709bfd8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -36,7 +36,7 @@
 
 before_install:
   # Install the appropriate Linux packages.
-  - if [[ "$VULKAN_BUILD_TARGET" == "LINUX" ]]; then sudo apt-get -y install libxkbcommon-dev libwayland-dev libmirclient-dev libxrandr-dev; fi
+  - if [[ "$VULKAN_BUILD_TARGET" == "LINUX" ]]; then sudo apt-get -y install libxkbcommon-dev libwayland-dev libmirclient-dev libxrandr-dev libx11-xcb-dev; fi
 
   # Install the Android NDK.
   - if [[ "$VULKAN_BUILD_TARGET" == "ANDROID" ]]; then export ARCH=`uname -m`; fi