can't read included file while parsing raml in subdir

Didn't fix it yet, just modified test to reveal the issue.

CLI `raml2html api/example.raml -o api/example.haml` works.
but test will fail 

```sh
  1) gulp-raml2html in buffer mode can convert an example RAML file:
     Uncaught Error: api/example.raml:215:12: Parse error while reading included.raml: cannot read included.raml (Error: ENOENT, open 'included.raml')
      at /Users/merlin/workspace/gulp-raml2html/index.js:26:16
      at process._tickCallback (node.js:415:13)
```

As per the [RAML spec](http://raml.org/spec.html)
> If a relative path is used for the included file, the path is interpreted relative to the location of the original (including) file. If the original file is fetched as an HTTP resource, the included file SHOULD be fetched over HTTP.