Set home directory for libreoffice conversion

Fixes https://github.com/owncloud/documents/issues/291#issuecomment-48953083

lib/converter.php

@@ -49,6 +49,7 @@ class Converter {
 		$params = ' --headless --convert-to ' . $targetFilter . ' --outdir ' 
 				. escapeshellarg($outdir) 
 				. ' --writer '. escapeshellarg($infile)
+				. ' -env:UserInstallation=file://' . get_temp_dir()
 		;
 		
 		file_put_contents($infile, $input);