add xmerl to applications list

[manukall]

this adds xmerl to the applications list. without it, xmerl is missing in (exrm-)releases.

[sorentwo]

Thanks, I'll release a patch version for this.

[mpranjic]

Hey guys,

I'm having issues making a release with mix release:

===> Provider (resolve_release) failed with: {error,
                                                     {rlx_prv_release,
                                                      {failed_solve,
                                                       {unreachable_package,
                                                        xmerl}}}}

==> ERROR: "Unable to find application xmerl. See the debug output for more information."

erl/OTP 19 and elixir 1.3.2

[mpranjic]

So I had to install erlang-xmerl (on ubuntu), issue solved.
(should we document it?)

[sorentwo]

I haven't experienced (or heard of) the issue on Ubuntu before. How was Erlang installed, from an apt repo or from Erlang Solutions? Either way, I'd welcome a note about it in the README.