#_ comments are not marked as comments (e.g. via `syntax-table` text property)

## Expected behavior

`#_` comments should mark the following expr as a comment (e.g. via `syntax-table` text property).

## Actual behavior

It does not happen.

## Steps to reproduce the problem

Enable rainbow-delimiters. See that the rainbow delimiters are not disabled
in the form following `#_`:

See the corresponding (invalid) issue in `rainbow-delimiters`: https://github.com/Fanael/rainbow-delimiters/issues/29

## Environment & Version information

### clojure-mode version information

```
clojure-mode (version 5.7.0-snapshot)
```

### Emacs version

```
GNU Emacs 25.1.90.1 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 10.11.6 (Build 15G1217))
of 2017-02-06
```

### Operating system

OS X 10.11