Refer forked xmerl to avoid many undef exceptions thrown

Original issue: #669 

Add forked xmerl to deps which refers https://github.com/shino/xmerl/pull/2 .
Xmerl source is based on Kelly's fork, updated to R16B02's and fetched the following change.

- Pull request to erlang/otp by Richard Carlsson: https://github.com/erlang/otp/pull/87

The PR was already merged to both maint and master.
Once new Erlang/OTP will be released (probably R16B03), we don't need forked one any more.