All OS's should use the same path separator

Path need to be consistence across all OS's. As it was before, when using this on Windows, path separator would be a backslash `\` while all other OS's would use `/`. Windows can read `/` as a directory separator so this won't be a problem. A standard `/`-separator will allow us to get the key with a standard syntax without some extra hack.

**Running gulp-rev from a Windows machine**
With this change, the `rev-manifest.json` file would be like this:

~~~
{
  "css/bootstrap.css": "css/bootstrap-12eb57ae.css"
}
~~~

Without my change, it would be like this:

~~~
{
  "css\\bootstrap.css": "css\\bootstrap-12eb57ae.css"
}
~~~