Add :castore to deps

[sgfn]

This fixes the following error when using secure?: true:

iex(2)> Jellyfish.Room.create(c)
** (RuntimeError) default CA trust store not available; please add `:castore` to your project's dependencies or specify the trust store using the :cacertfile/:cacerts option within :transport_options. From OTP 25, you can also use:

  * :public_key.cacerts_get/0 to get certificates that you loaded from files or
  * from the OS with :public_key.cacerts_load/0,1

See: https://www.erlang.org/blog/my-otp-25-highlights/#ca-certificates-can-be-fetched-from-the-os-standard-place

    (mint 1.5.1) lib/mint/core/transport/ssl.ex:689: Mint.Core.Transport.SSL.raise_on_missing_castore!/0
    (mint 1.5.1) lib/mint/core/transport/ssl.ex:557: Mint.Core.Transport.SSL.add_cacerts/1
    (mint 1.5.1) lib/mint/core/transport/ssl.ex:442: Mint.Core.Transport.SSL.add_verify_opts/2
    (mint 1.5.1) lib/mint/core/transport/ssl.ex:432: Mint.Core.Transport.SSL.ssl_opts/2
    (mint 1.5.1) lib/mint/core/transport/ssl.ex:328: Mint.Core.Transport.SSL.connect/4
    (mint 1.5.1) lib/mint/negotiate.ex:80: Mint.Negotiate.connect_negotiate/4
    (tesla 1.7.0) lib/tesla/adapter/mint.ex:159: Tesla.Adapter.Mint.open_conn/2
    iex:2: (file)

[codecov]

Codecov Report

Merging #32 (6952665) into master (508e96e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   85.20%   85.20%           
=======================================
  Files          11       11           
  Lines         169      169           
=======================================
  Hits          144      144           
  Misses         25       25           

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