Need version guard on colors.lua

01c9bd723f2631971b2c8cda8f8ea037221d5149 introduced the following snippet

```
if has('nvim')
  source $VIMRUNTIME/colors/vim.lua
else
  hi clear
endif
```

Debian is behind on nvim (still at 0.9.5) so this needs a version guard around it as that file is not available yet.