Travis CI: change to docker infrastructure.
Travis CI: change to docker infrastructure.

antiagainst/shaderc,antiagainst/shaderc,AWoloszyn/shaderc,dneto0/shaderc,antiagainst/shaderc,dneto0/shaderc,fuchsia-mirror/third_party-shaderc,Qining/shaderc,fuchsia-mirror/third_party-shaderc,fuchsia-mirror/third_party-shaderc,dneto0/shaderc,Qining/shaderc,AWoloszyn/shaderc,dneto0/shaderc,AWoloszyn/shaderc,Qining/shaderc,antiagainst/shaderc