Add pkpass (Apple Passbook) to list of Mime-Types (#8)

.pkpass is the official extension from apple for passbook (apple wallet).

Documentation:
https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/PassKit_PG/DistributingPasses.html

Author: Philipp Rappold

src/ApacheMimetypeHelper.php

@@ -91,6 +91,7 @@ public function getMimetypeFromExtension($extension)
             'pbm' => 'image/x-portable-bitmap',
             'pdf' => 'application/pdf',
             'pgm' => 'image/x-portable-graymap',
+            'pkpass' => 'application/vnd.apple.pkpass',
             'png' => 'image/png',
             'pnm' => 'image/x-portable-anymap',
             'ppm' => 'image/x-portable-pixmap',