remove mix.lock file

msgpack-elixir is a library, and not a top level application. The
mix lockfile should not be included in library dependencies. See
the following article for a more information:

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
bump dependencies to elixir 13 compatible versions
add hex.pm packaging information to mix.exs
Merge pull request #3 from reset/upgrade-elixir-13

Upgrade to Elixir 13