Change app version to 2.0.0, to avoid issues on Heroku

I was having pushing an application to Heroku, using the [Elixir buildpack](https://github.com/goshakkk/heroku-buildpack-elixir) and depending on Jsonex. 

    could not compile dependency jsonex, mix compile failed. You can recompile this dependency with `mix deps.c>
    ** (Mix) Expected :version to be a SemVer version

Adding the patch number to the version fixes the compilation issue, and does seem to make it compatible with http://semver.org.

Thanks.