docs: Fix README to show Travis master br status
docs: Fix README to show Travis master br status

Better to show status only for master branch Travis builds.  Other branches frequently fail and cause the badge to not reflect the status of the master branch.
KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools