Fixing disallowMultipleLineBreaks when file starts with #!

Version 1.5.3 introduced a change that breaks disallowMultipleLineBreaks in files that have a line like:

```
#! /usr/bin/env node
```

There are already tests for string-checker.js for this here: https://github.com/mdevils/node-jscs/blob/master/test/string-checker.js#L12

... but those tests apparently don't cover the issue I'm seeing.  I'm a bit worried my regex change will break something else, but all the tests still pass.

I've also verified that this change works in my real code projects.