make TikaMW file access compatible with MediaWiki 1.21

MediaWiki changed their API for getting stored files.  As of
5275f9b097cdae5dbab0845e3ea127086e3a6570 on mediawiki/core,
File::getPath() no longer returns a path on the physical
file system.  Instead, it uses mwstore:// paths, which when
passed to PHP functions like file_exists() and file_get_contents(),
causes warnings.  The fix is to use File::exists() and 
File::getLocalRefPath() instead.  The rationale can be verified
by examining this diff:

https://git.wikimedia.org/commitdiff/mediawiki%2Fcore/5275f9b097cdae5dbab0845e3ea127086e3a6570

wherein one can see the old API calls getting replaced
with the new API.