Allow hex in mb_encode_numericentity

Summary: PHP 5.4 added an additional `is_hex` parameter to `mb_encode_numericentity`. We should
support it now given that we have also just updated our `libmbfl` library.

Fixes #4247

Reviewed By: @sgolemon

Differential Revision: D1691911

Signature: t1:1691911:1418418614:0e3f630d326c3a7488be152bea1dd1e3df7224ba