Prevent backslashes issues on Windows

Hello,

I got an issue on Windows, the result of the `path.join()` was `'.grunt\\grunt-contrib-jasmine\\grunt-template-jasmine-istanbul\\reporter.js'`.

Using it with **grunt-template-jasmine-require** it fails with the following error:

```
>> Error: scripterror: Illegal path or script error: 
['.gruntgrunt-contrib-jasminegrunt-template-jasmine-istanbulreporter.js']
```

This little hotfix will do the job properly (it works \o/).