Update argon2_elixir

[leifmetcalf]

Hey, not really sure what I'm doing but I tried bumping the version of argon2_elixir from ~> 3.0 to ~> 4.0. The changes I see on https://github.com/riverrun/argon2_elixir/compare/v3.2.1...v4.1.0 are to:

• change the default difficulty options
• replace uses of '...' by ~c"...".

Some other dependencies in integration_test/mix.lock were updated incidentally. The tests passed, so I guess this is alright?

[josevalim]

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