Check for single quotes on import statements

When checking if the ``@import`` argument url is quoted, it [currently only checks for double quotes.](https://github.com/mgreter/sass2scss/blob/a35b799f49d7d335ddc2b765b13d7d5db1d1247f/sass2scss.cpp#L572). I'm not sure if this is part of the official spec but libsass supports single quotes for imports in scss files.

So before when compiling this:
```sass
@import 'bourbon'

div
  @include border-radius(10px)
```

It resolved to something like this:

```scss
@import "'bourbon'";
div { @include border-radius(10px) }
```

``bourbon`` ended up surrounded by both single and double quotes.

I just changed the check to also look for single quotes when determining if the import argument is quoted.