Travis CI: bump installed wayland-protocols to v1.15

Move away from v1.12 as GLFW now requires >= 1.15.

.travis.yml

@@ -118,7 +118,7 @@ script:
       sudo dpkg -i extra-cmake-modules_5.38.0a-0ubuntu1_amd64.deb;
       git clone git://anongit.freedesktop.org/wayland/wayland-protocols;
       pushd wayland-protocols;
-      git checkout 1.12 && ./autogen.sh --prefix=/usr && make && sudo make install;
+      git checkout 1.15 && ./autogen.sh --prefix=/usr && make && sudo make install;
       popd;
     fi
   - $RUNNER cmake $CMAKE_ARCH_ARGS