Forces oauther library version that deals with OTP 24

The newest versions of OTP do not have `:crypto.hmac/3` anymore and this
has broken TIL integration with Twitter since the last big app bump to
use OTP 24. `extwitter` library uses `oauther` which does not have
changed the `:crypto.mac/4` yet, so we're going to use this hot fix
branch for now.

For reference:
lexmag/oauther#22
parroty/extwitter#140

mix.exs

@@ -55,6 +55,8 @@ defmodule Tilex.Mixfile do
       {:hackney, "1.17.4"},
       {:html_sanitize_ex, "~> 1.2"},
       {:jason, "~> 1.0"},
+      {:oauther,
+       git: "https://github.com/tobstarr/oauther.git", branch: "master", override: true},
       {:optimus, "~> 0.2.0"},
       {:phoenix, "~> 1.5.9"},
       {:phoenix_ecto, "~> 4.3.0"},

mix.lock

@@ -38,7 +38,7 @@
   "mimerl": {:hex, :mimerl, "1.2.0", "67e2d3f571088d5cfd3e550c383094b47159f3eee8ffa08e64106cdf5e981be3", [:rebar3], [], "hexpm", "f278585650aa581986264638ebf698f8bb19df297f66ad91b18910dfc6e19323"},
   "mochiweb": {:hex, :mochiweb, "2.20.1", "e4dbd0ed716f076366ecf62ada5755a844e1d95c781e8c77df1d4114be868cdf", [:rebar3], [], "hexpm", "d1aeee7870470d2fa9eae0b3d5ab6c33801aa2d82b10e9dade885c5c921b36aa"},
   "oauth2": {:hex, :oauth2, "2.0.0", "338382079fe16c514420fa218b0903f8ad2d4bfc0ad0c9f988867dfa246731b0", [:mix], [{:hackney, "~> 1.13", [hex: :hackney, repo: "hexpm", optional: false]}], "hexpm", "881b8364ac7385f9fddc7949379cbe3f7081da37233a1aa7aab844670a91e7e7"},
-  "oauther": {:hex, :oauther, "1.1.1", "7d8b16167bb587ecbcddd3f8792beb9ec3e7b65c1f8ebd86b8dd25318d535752", [:mix], [], "hexpm", "9374f4302045321874cccdc57eb975893643bd69c3b22bf1312dab5f06e5788e"},
+  "oauther": {:git, "https://github.com/tobstarr/oauther.git", "e81fc6588e52eeaf41cdf51ed9d968da46b0b67d", [branch: "master"]},
   "optimus": {:hex, :optimus, "0.2.0", "3e868b8bedc723d70189cde715df3ef804524c55b26fb33e89cd0f691f50fe80", [:mix], [], "hexpm", "95bcbf228d4ba67283747629087cc3650b10d24390e4ea533b0a64ca278c181c"},
   "parse_trans": {:hex, :parse_trans, "3.3.1", "16328ab840cc09919bd10dab29e431da3af9e9e7e7e6f0089dd5a2d2820011d8", [:rebar3], [], "hexpm", "07cd9577885f56362d414e8c4c4e6bdf10d43a8767abb92d24cbe8b24c54888b"},
   "phoenix": {:hex, :phoenix, "1.5.9", "a6368d36cfd59d917b37c44386e01315bc89f7609a10a45a22f47c007edf2597", [:mix], [{:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:phoenix_html, "~> 2.13 or ~> 3.0", [hex: :phoenix_html, repo: "hexpm", optional: true]}, {:phoenix_pubsub, "~> 2.0", [hex: :phoenix_pubsub, repo: "hexpm", optional: false]}, {:plug, "~> 1.10", [hex: :plug, repo: "hexpm", optional: false]}, {:plug_cowboy, "~> 1.0 or ~> 2.2", [hex: :plug_cowboy, repo: "hexpm", optional: true]}, {:plug_crypto, "~> 1.1.2 or ~> 1.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "7e4bce20a67c012f1fbb0af90e5da49fa7bf0d34e3a067795703b74aef75427d"},