Specify encoding of source files as utf-8

Per
http://pear.php.net/manual/en/package.php.php-codesniffer.advanced-usage.php.

Shell script wrapper update is: https://gerrit.wikimedia.org/r/88745

Bug: 55508
Change-Id: I44f42262d709b2b9ad876bdf9fc44fb5e3dfff09