Fix the test for built-in models on Windows

/to @bajtos 

The PR fixes how built-in models are checked on Windows where the file path separator is NOT /. 

Please note no unit test is added because one of the existing tests is failing on Windows before this patch.

See https://github.com/strongloop/loopback/issues/756
This solution seems unnecessary complex and low-performant to me. How about using the path separator to build the last regex? This regex can be also prepared once at the load time and stored in a module-global constant.

```
// at the top
LOOPBACK_MODEL_REGEXP = new RegExp(
  ['node_modules', 'loopback', '[^\/\\]+', 'models', '[^\/\\]+\.js$']
    .join('\\' + path.sep));

// in the function
return data.sourceFile && LOOPBACK_MODEL_REGEXP.test(data.sourceFile);
```