Update YamlFileLoader.php

getPackagePath should really a string not an array, so i have added the file extension as a parameter and using it like so in cascadePackage so this solves the error i was getting "file_exists() expects parameter 1 to be a valid path, array given"
Merge pull request #2 from unitedworx/master

getPackagePath should only return a string. Fix #2.