Support CJK scenario label.

Label is removed reference filename when use japanese character in scenario's label.

backstop.json

```
{
"id": "cjk_test",
"viewports": [
{
"name": "viewport",
"width": 568,
"height": 1024
}
],
"scenarios": [
{
"label": "hello",
"url": "index.html"
},
{
"label": "こんにちは",
"url": "index.html"
}
],
"paths": {
"bitmaps_reference": "backstop_data/bitmaps_reference",
"bitmaps_test": "backstop_data/bitmaps_test",
"casper_scripts": "backstop_data/casper_scripts",
"html_report": "backstop_data/html_report",
"ci_report": "backstop_data/ci_report"
},
"casperFlags": [],
"engine": "casper",
"report": ["browser"],
"debug": false
}
```

Generate reference filename

* cjk_test__0_document_0_viewport.png
* cjk_test_hello_0_document_0_viewport.png

Removing `.replace(/[^a-z0-9_-]/gi, '')` from `function getFileName` will work correctly.

https://github.com/garris/BackstopJS/blob/master/core/util/runChromy.js#L542