travis: Add OSX meson build to matrix

Unfortunately, a 'brew update' is currently needed to get the meson >=
0.46.0 required by xserver.

.travis.yml

@@ -1,18 +1,27 @@
 language: c
-cache: ccache
+cache:
+    ccache: true
+    directories:
+        - $HOME/Library/Caches/Homebrew
 branches:
     except:
         - /appveyor.*/
 
+os: osx
+osx_image: xcode9.2
+
 matrix:
   include:
-    - os: osx
-      osx_image: xcode9.2
-      env: DISTRO=xcode9.2
+    - env: TOOL=meson
+    - env: TOOL=autotools
 
 install:
-    - HOMEBREW_NO_AUTO_UPDATE=1 brew install ccache
+    - brew update
+    - HOMEBREW_NO_AUTO_UPDATE=1 brew install ccache meson
 
 script:
-    - ./test/scripts/build-travis-osx.sh
+    - ./test/scripts/build-travis-osx.sh $TOOL
     - ccache -s
+
+before_cache:
+    - brew cleanup