build: Adjust travis config for VT updates
build: Adjust travis config for VT updates

- VT update external script no longer accepts jsoncpp flag
  so remove call to build it.  The script now builds it all the time.
- Add back cmake flag to turn off LVL layers build.  LVL should
  now build the layer utils even with layer build disabled.  VT
  needs the layer utils to build devsim.

Change-Id: I1c5421d4b5553b3d39f68f075ee5850588696775

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