Consistent semantic versioning

Unfortunately, when I went to switch datomex to use the hex version, I ran into this error:

```
$ mix test
==> erldn (compile)
** (Mix) The application erldn specified a non Semantic Version 1.0. Mix can only match the requirement ~> 1.0 against Semantic Versions, to match against any version, please use a regex as requirement
```

@josevalim on irc pointed me to the `app.src` file, so I took a look at how other erlang projects published on hex solved this issue.

Introduces a VERSION file as the single source of truth for version, so
that inconsistencies between the package.exs and src/erldn.app.src don't
repeat themselves.

Thanks again for working with me on this. Looking forward to bumping datomex as soon as this is pushed up to hex.pm.