Travis: Disable OSX builds, they create too much backlog

Travis CI has not enough OSX build nodes allocated for open source projects,
so we often have to wait several hours (up to a full day) for builds to start.

(cherry picked from commit e0c00efb99)

.travis.yml

@@ -28,9 +28,9 @@ matrix:
     - env: GODOT_TARGET=android TOOLS=no CACHE_NAME=${GODOT_TARGET}-gcc
       os: linux
       compiler: gcc
-    - env: GODOT_TARGET=osx TOOLS=yes CACHE_NAME=${GODOT_TARGET}-clang-tools
-      os: osx
-      compiler: clang
+    #- env: GODOT_TARGET=osx TOOLS=yes CACHE_NAME=${GODOT_TARGET}-clang-tools
+    #  os: osx
+    #  compiler: clang
     #- env: GODOT_TARGET=iphone TOOLS=no CACHE_NAME=${GODOT_TARGET}-clang
     #  os: osx
     #  compiler: clang