Update Elixir dep to v0.13-dev

[darkofabijan]

With latest mix.lock update Phoenix requires v0.13 version. Project template is also updated.

[rtwomey]

👍, as I was just bitten by this in the form of the following (on Elixir 0.12.5, which is also the most up-to-date version via homebrew):

** (SyntaxError) nofile:1: invalid token: %{"cowboy": {:git, "git://github.com/extend/cowboy.git", "05024529679d1d0203b8dcd6e2932cc2a526d370", []},
    (elixir) src/elixir.erl:113: :elixir.eval/3
    (elixir) lib/code.ex:130: Code.do_eval_string/3
    (mix) lib/mix/deps/lock.ex:58: Mix.Deps.Lock.read/0
    (mix) lib/mix/tasks/deps.get.ex:22: Mix.Tasks.Deps.Get.run/1
    (mix) lib/mix/cli.ex:59: Mix.CLI.run_task/2
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/2

I was also able to just delete mix.lock and run mix deps.get to regenerate the lockfile, though not sure if anything else relies on 0.13-dev yet.