[bug] CSS link tag trailing slash

Let's say I have:

```
<!-- build:css build/css/app.min.css -->
<link rel="stylesheet" href="/assets/third-party/bootstrap/dist/css/bootstrap.min.css">
<!-- endbuild -->
```
which outputs:

```
<link rel="stylesheet" href="build/css/app.min.css"/>
```
This is invalid HTML because the link tag should not contain a trailing slash. I'm running into this bug trying to run gulp-prefix on the link tags after useref completes, but gulp-prefix bails on the link tags created by gulp-useref on account of this trailing slash.

My original gulp-useref [issue](https://github.com/jonkemp/gulp-useref/issues/35#issuecomment-47261753) was redirected here, and this PR provides a simple fix to make the outputted link tag conform to the HTML5 spec.