@supports query breaks highlighting

Minimal `.vimrc`:

```vim
set nocompatible
set runtimepath+=~/.vim/bundle/vim-haml
filetype plugin on
filetype indent on
syntax on
```

Example:

<img width="274" alt="Screen Shot 2019-05-15 at 7 17 57 PM" src="https://user-images.githubusercontent.com/643417/57815623-298c7e00-7746-11e9-95fb-d86d6b1ba3f6.png">
