travis: update to 18.04, as it is the earliest supportef by vulkan sdk

.travis.yml

@@ -34,7 +34,7 @@ jobs:
       name: "Build for Linux i686"
       cache: ccache
       os: linux
-      dist: xenial
+      dist: bionic
       addons:
         apt:
           packages:
@@ -125,7 +125,7 @@ jobs:
       name: "Build for Linux amd64"
       cache: ccache
       os: linux
-      dist: xenial
+      dist: bionic
       addons:
         apt:
           packages:

scripts/travis_linux_deps.sh

@@ -5,7 +5,7 @@ mv SDL2-$SDL_VERSION SDL2_src
 
 # ref_vk required Vulkan SDK
 wget -qO - https://packages.lunarg.com/lunarg-signing-key-pub.asc | sudo apt-key add -
-sudo wget -qO /etc/apt/sources.list.d/lunarg-vulkan-focal.list https://packages.lunarg.com/vulkan/lunarg-vulkan-focal.list
+sudo wget -qO /etc/apt/sources.list.d/lunarg-vulkan-1.2.176-bionic.list https://packages.lunarg.com/vulkan/1.2.176/lunarg-vulkan-1.2.176-bionic.list
 sudo apt update
 [ "$ARCH" = "i386" ] && SUFFIX=":i386" || SUFFIX=""
 sudo apt install -y vulkan-sdk"$SUFFIX"