Fix Elixir 1.5 deprecation warnings

Kernel.to_char_list/1 was soft-deprecated in Elixir 1.3, which means it
didn't actually issue any warnings at the time. Circa Elixir 1.5, it
actually produces warnings. Since mix.exs is pointing to ~> 1.3, where
both names are available, it should be fine to just go ahead and use
Kernel.to_charlist/1.

Also updates the mix.lock file with the latest dependencies, which
hadn't been part of the commit in #5.