Fix single file builds.

The recent changes broke single output file builds. Version `0.1.2` worked fine. After some debugging, the culprit is https://github.com/guybedford/require-css/commit/ba7680c04f9bb27cdcc38a9b11b0f7f63b5b8cf0

The "data" object contains a "path" property containing the full path of the output file (already normalised) for both single and multiple files. Having this said, there's no need to check for `config.dir`.
Also the `mkdirSync` is unnecessary because requirejs already creates the output directory.

Cleaned leading/trailing whitespaces