Ignore UTF8 byte order mark when linting for license header.

Summary: This makes the license linter automatically strip the UTF8 BOM if it exists in a file.

Test Plan: Ran `arc lint CrashReport/Program.cs` and it no longer complained.

Reviewers: Sabre070

Reviewed By: Sabre070

CC: buildbot

Differential Revision: http://code.redpointsoftware.com.au/D111