Use -O encoding=utf-8 for pygmentize

Summary:
Explicitly specify UTF-8 encoding, as we use it internally and it resolves at least one real-world problem @auduny ran into.

The test case ("tettpånett") works fine for me with or without this change, but I'm guessing my pygments defaults to utf8 for some reason. It seems like making the encoding we expect explicit can't hurt anything.

Test Plan:

• Looked at some highlighted files on my system, everything worked properly.
• @auduny reported that this resolved an issue he encountered.

Reviewers: auduny, btrahan, vrana

Reviewed By: auduny

CC: aran

Differential Revision: https://secure.phabricator.com/D2617