Update the format in mix.lock

Actually, I just did a `mix deps.get`, which triggered an update of Hex's syntax which now uses `:hex`, `:git` and similar tags instead of `:package`. I don't know if this can cause problems of some sort, but I figured a short PR couldn't hurt.

**PS** I tested this on Elixir 1.0.0 and 1.0.2.