Update project slick to 1.3.15

libgrabber updated project slick to 1.3.15.

@kenwheeler

### Metadata information

```json
{
  "metadata": {
    "packageManager": "github",
    "name": "slick",
    "repo": "kenwheeler/slick",
    "files": {
      "basePath": "slick/",
      "include": ["slick.min.js", "slick.css", "ajax-loader.gif", "./fonts/**/*"],
      "exclude": []
    },
    "path": "/home/jsdelivr/files/jquery.slick",
    "localVersions": ["1.3.2", "1.3.4", "1.3.6", "1.3.7", "1.3.8", "1.3.9", "1.3.10", "1.3.11", "1.3.12", "1.3.13", "1.3.14"],
    "remoteVersions": ["1.1.3", "1.2.0", "1.2", "1.2.1", "1.2.2", "1.2.3", "1.2.4", "1.2.5", "1.2.6", "1.2.7", "1.2.8", "1.2.9", "1.2.10", "1.3.0", "1.3.1", "1.3.2", "1.3.3", "1.3.4", "1.3.5", "1.3.6", "1.3.7", "1.3.8", "1.3.9", "1.3.10", "1.3.11", "1.3.12", "1.3.13", "1.3.14", "1.3.15"],
    "branchVersions": []
  },
  "version": "1.3.15",
  "updated": true,
  "updatePath": "/home/jsdelivr/files/jquery.slick/1.3.15",
  "branch": "slick/1.3.15"
}
```