Refer forked xmerl to avoid many undef exceptions thrown

The xmerl fork has change made by pull request on erlang/otp:
https://github.com/erlang/otp/pull/87 (already merged to maint and master)

Once Erlang/OTP is released, we don't need forked one any more.
At that time we will remove xmerl from deps and use one in OTP.