Prioritize parent node_module tree in nodeModulesPathsSync before fallback options.paths

Hi James. According to http://nodejs.org/docs/latest/api/all.html#all_loading_from_the_global_folders, 
```If the NODE_PATH environment variable is set to a colon-delimited list of absolute paths, then node will search those paths for modules if they are not found elsewhere.```

Yet the implementation in `index.js` of the pertinent function, `nodeModulesPathsSync`, concatenated the list of node_module path combinations to the end of the NODE_PATH isomorphic options.dirs: this seems backwards, no?