ensure it isn't a local file before appending http

It's important to note we're using [phantomjs' exists
method](https://github.com/ariya/phantomjs/wiki/API-Reference-FileSystem#wiki-filesystem-query-functions)
here and not [nodejs' exists
method](http://nodejs.org/api/fs.html#fs_fs_exists_path_callback).  I
was a little confused at first :)