[mustache.php] Fixes is_dir not correctly handling 'file://' in filename

Should probably be merged with  #1732 , but they deal with different fixes, so I decided to stick with the one feature per pull paradigm.

The 'file://' stripping I found in https://github.com/facebook/hhvm/blob/master/hphp/runtime/base/file-stream-wrapper.cpp#L47 , not sure if file-stream-wrapper usage would be more appropriate in this context.

Fixes #1835