build(serum): pin earmark to v1.4.4

The latest version of Earmark at this moment (v1.4.5)
prevents the TOC generator plugin from working properly.

mix.exs

@@ -36,7 +36,7 @@ defmodule Serum.Mixfile do
   defp deps do
     [
       {:serum_sdk, sdk_spec(Mix.env())},
-      {:earmark, "~> 1.3"},
+      {:earmark, "1.4.4"},
       {:file_system, "~> 0.2"},
       {:microscope, "~> 1.3"},
       {:timex, "~> 3.5"},

mix.lock

@@ -6,7 +6,7 @@
   "cowlib": {:hex, :cowlib, "2.7.3", "a7ffcd0917e6d50b4d5fb28e9e2085a0ceb3c97dea310505f7460ff5ed764ce9", [:rebar3], [], "hexpm", "1e1a3d176d52daebbecbbcdfd27c27726076567905c2a9d7398c54da9d225761"},
   "credo": {:hex, :credo, "1.3.2", "08d456dcf3c24da162d02953fb07267e444469d8dad3a2ae47794938ea467b3a", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "b11d28cce1f1f399dddffd42d8e21dcad783309e230f84b70267b1a5546468b6"},
   "dialyxir": {:hex, :dialyxir, "1.0.0", "6a1fa629f7881a9f5aaf3a78f094b2a51a0357c843871b8bc98824e7342d00a5", [:mix], [{:erlex, ">= 0.2.6", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "aeb06588145fac14ca08d8061a142d52753dbc2cf7f0d00fc1013f53f8654654"},
-  "earmark": {:hex, :earmark, "1.4.3", "364ca2e9710f6bff494117dbbd53880d84bebb692dafc3a78eb50aa3183f2bfd", [:mix], [], "hexpm", "8cf8a291ebf1c7b9539e3cddb19e9cef066c2441b1640f13c34c1d3cfc825fec"},
+  "earmark": {:hex, :earmark, "1.4.4", "4821b8d05cda507189d51f2caeef370cf1e18ca5d7dfb7d31e9cafe6688106a4", [:mix], [], "hexpm", "1f93aba7340574847c0f609da787f0d79efcab51b044bb6e242cae5aca9d264d"},
   "erlex": {:hex, :erlex, "0.2.6", "c7987d15e899c7a2f34f5420d2a2ea0d659682c06ac607572df55a43753aa12e", [:mix], [], "hexpm", "2ed2e25711feb44d52b17d2780eabf998452f6efda104877a3881c2f8c0c0c75"},
   "ex_doc": {:hex, :ex_doc, "0.21.3", "857ec876b35a587c5d9148a2512e952e24c24345552259464b98bfbb883c7b42", [:mix], [{:earmark, "~> 1.4", [hex: :earmark, repo: "hexpm", optional: false]}, {:makeup_elixir, "~> 0.14", [hex: :makeup_elixir, repo: "hexpm", optional: false]}], "hexpm", "0db1ee8d1547ab4877c5b5dffc6604ef9454e189928d5ba8967d4a58a801f161"},
   "ex_json_schema": {:hex, :ex_json_schema, "0.5.6", "9a96edd51a00fc005e12a773a7a79e507a77855e69fcb3d9a02f7f39c3265c03", [:mix], [], "hexpm"},