build: Compile with both clang and gcc on Travis
build: Compile with both clang and gcc on Travis

Fixes #1310

Change-Id: I430089a362b70e104ad3274a76b1033b2e94ac2a

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers