migrate to container-based Travis CI

(cherry picked from commit 1f83ae9b5a)

.travis.yml

@@ -1,7 +1,6 @@
 language: cpp
 
-sudo: required
+sudo: false
-dist: trusty
 
 compiler:
   - gcc
@@ -41,10 +40,33 @@ matrix:
     - compiler: clang
       env: GODOT_TARGET=x11
 
+addons:
+  apt:
+    packages:
+      - build-essential
+      - scons
+      - pkg-config
+      - libx11-dev
+      - libxcursor-dev
+      - libasound2-dev
+      - libfreetype6-dev
+      - libgl1-mesa-dev
+      - libglu1-mesa-dev
+      - libssl-dev
+      - libxinerama-dev
+      - libudev-dev
+
+      # For cross-compiling to Windows.
+      - binutils-mingw-w64-i686
+      - binutils-mingw-w64-x86-64
+      - gcc-mingw-w64-i686
+      - gcc-mingw-w64-x86-64
+      - g++-mingw-w64-i686
+      - g++-mingw-w64-x86-64
+      - mingw-w64
+
 
 before_script:
-  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get update -qq; sudo apt-get install -y scons pkg-config libx11-dev libxcursor-dev build-essential libasound2-dev libfreetype6-dev libgl1-mesa-dev libglu-dev libssl-dev libxinerama-dev libudev-dev; fi
-  - if [ "$TRAVIS_OS_NAME" = "linux" ] && [ "$GODOT_TARGET" = "windows" ]; then sudo apt-get update -qq; sudo apt-get install -y mingw32 mingw-w64; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then brew update; brew install scons; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" ] && [ "$GODOT_TARGET" = "android" ]; then brew update; brew install android-sdk android-ndk; export ANDROID_HOME=/usr/local/opt/android-sdk; export ANDROID_NDK_ROOT=/usr/local/opt/android-ndk; fi