highlight raw string literal

I'm using vim 7.4 shipped with Ubuntu 14.04 LTS. I don't get correct highlight of raw string literals:

![screenshot from 2014-10-23 21 29 22](https://cloud.githubusercontent.com/assets/3337209/4753694/2aef797c-5ab9-11e4-9020-322f6b56bd57.png)

After this patch applied, things look like:

![screenshot from 2014-10-23 21 34 34](https://cloud.githubusercontent.com/assets/3337209/4753862/17a2a4ba-5aba-11e4-8c85-e00b674f978e.png)