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.
Remove xmerl from dialyzer apps

Since we're using a fork of xmerl, this prevents an error like:

```
==> riak_cs (dialyzer)

dialyzer: Could not merge PLTs since they are not disjoint
The following files are included in more than one PLTs:
["xmerl.beam","xmerl_b64Bin.beam","xmerl_b64Bin_scan.beam",
 "xmerl_eventp.beam","xmerl_html.beam","xmerl_lib.beam","xmerl_otpsgml.beam",
 "xmerl_regexp.beam","xmerl_sax_old_dom.beam","xmerl_sax_parser.beam",
 "xmerl_sax_parser_latin1.beam","xmerl_sax_parser_list.beam",
 "xmerl_sax_parser_utf16be.beam","xmerl_sax_parser_utf16le.beam",
 "xmerl_sax_parser_utf8.beam","xmerl_sax_simple_dom.beam","xmerl_scan.beam",
 "xmerl_sgml.beam","xmerl_simple.beam","xmerl_text.beam","xmerl_ucs.beam",
 "xmerl_uri.beam","xmerl_validate.beam","xmerl_xlate.beam","xmerl_xml.beam",
 "xmerl_xpath.beam","xmerl_xpath_lib.beam","xmerl_xpath_parse.beam",
 "xmerl_xpath_pred.beam","xmerl_xpath_scan.beam","xmerl_xs.beam",
 "xmerl_xsd.beam","xmerl_xsd_type.beam"]

make: *** [dialyzer] Error 1
```

You will likely need to `make cleanplt` in order to rebuild the PLT.
Merge pull request #848 from basho/feature/xmerl-avoid-undef-rebased

Feature/xmerl avoid undef rebased

Reviewed-by: kuenishi