Pygments: Also treat .mm files as Objective-C

Summary:
Objective-C files come in two flavors (.m for Objective-C,
.mm for Objective-C++). This updates the Pygments syntax
highlighter to handle Objective-C++ files.

Test Plan: Inspection.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1569