cursorline support sometimes leaves behind spurious highlighting

edit: happens after jumping from a css file to a ruby file

<img width="208" alt="screen shot 2017-10-20 at 12 32 21" src="https://user-images.githubusercontent.com/1770351/31814576-c2f72fe4-b592-11e7-8942-f1c8d8c00310.png">

```
$ mvim --version
VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Oct  5 2017 04:42:50)
MacOS X (unix) version
Included patches: 1-1175
```