Here’s the error output:

→ mix deps.get
warning: the Collectable protocol is deprecated for non-empty lists. The behaviour of things like Enum.into/2 or "for" comprehensions with an :into option is incorrect when collecting into non-empty lists. If you're collecting into a non-empty keyword list, consider using Keyword.merge/2 instead. If you're collecting into a non-empty list, consider concatenating the two lists with the ++ operator.
  (elixir) lib/collectable.ex:83: Collectable.List.into/1
  (hex) lib/hex/remote_converger.ex:265: Hex.RemoteConverger.prepare_locked/3
  (hex) lib/hex/remote_converger.ex:36: Hex.RemoteConverger.converge/2
  (mix) lib/mix/dep/converger.ex:95: Mix.Dep.Converger.all/4
  (mix) lib/mix/dep/converger.ex:51: Mix.Dep.Converger.converge/4

Failed to fetch record for 'phoenix_pubsub' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'phoenix' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'phoenix_html' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'phoenix_live_reload' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'gettext' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'ecto_sql' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to fetch record for 'postgrex' from registry (using cache)
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}
Failed to check for new Hex version
{:failed_connect, [{:to_address, {'repo.hex.pm', 443}}, {:inet, [:inet], {:option, :server_only, :honor_cipher_order}}]}

The command elixir -v runs fine:

elixir -v
Erlang/OTP 22 [erts-10.6.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [dtrace]

Elixir 1.9.4 (compiled with Erlang/OTP 22)