Switch to Xenial build environment (#4813)

* Switch to Xenial build environment
* Add Carla submodule/weak linking support, related #3963 
* Fix Carla detection in AppImage, closes #5369

.travis.yml

@@ -1,6 +1,6 @@
 language: cpp
 compiler: gcc
-dist: trusty
+dist: xenial
 sudo: required
 cache:
     directories:
@@ -10,13 +10,17 @@ cache:
 matrix:
     include:
         - env: TARGET_OS=win32
+          dist: trusty
         - env: TARGET_OS=win64
+          dist: trusty
         - os: osx
           osx_image: xcode8.2
         - env: QT5=
         - env: QT5=True
         - env: QT5=True TARGET_OS=win32 TARGET_DEPLOY=True
+          dist: trusty
         - env: QT5=True TARGET_OS=win64 TARGET_DEPLOY=True
+          dist: trusty
         - os: osx
           osx_image: xcode8.2
           env: QT5=True