Add preferred_cli_env to mix file

mix.exs

@@ -10,6 +10,7 @@ defmodule Postgrex.Mixfile do
      deps: deps(),
      name: "Postgrex",
      source_url: "https://github.com/jumpn/postgrex",
+     preferred_cli_env: ["docs": :docs],
      docs: [source_ref: "v#{@version}", main: "readme", extras: ["README.md"]],
      description: description(),
      package: package()]

mix.lock

@@ -1,5 +1,5 @@
-%{"connection": {:hex, :connection, "1.0.4", "a1cae72211f0eef17705aaededacac3eb30e6625b04a6117c1b2db6ace7d5976", [:mix], [], "hexpm"},
-  "db_connection": {:hex, :db_connection, "1.1.0", "b2b88db6d7d12f99997b584d09fad98e560b817a20dab6a526830e339f54cdb3", [:mix], [{:connection, "~> 1.0.2", [hex: :connection, repo: "hexpm", optional: false]}, {:poolboy, "~> 1.5", [hex: :poolboy, repo: "hexpm", optional: true]}, {:sbroker, "~> 1.0", [hex: :sbroker, repo: "hexpm", optional: true]}], "hexpm"},
-  "decimal": {:hex, :decimal, "1.3.1", "157b3cedb2bfcb5359372a7766dd7a41091ad34578296e951f58a946fcab49c6", [:mix], [], "hexpm"},
-  "earmark": {:hex, :earmark, "1.2.2", "f718159d6b65068e8daeef709ccddae5f7fdc770707d82e7d126f584cd925b74", [:mix], [], "hexpm"},
-  "ex_doc": {:hex, :ex_doc, "0.16.1", "b4b8a23602b4ce0e9a5a960a81260d1f7b29635b9652c67e95b0c2f7ccee5e81", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"}}
+%{"connection": {:hex, :connection, "1.0.4", "a1cae72211f0eef17705aaededacac3eb30e6625b04a6117c1b2db6ace7d5976", [:mix], []},
+  "db_connection": {:hex, :db_connection, "1.1.0", "b2b88db6d7d12f99997b584d09fad98e560b817a20dab6a526830e339f54cdb3", [:mix], [{:connection, "~> 1.0.2", [hex: :connection, optional: false]}, {:poolboy, "~> 1.5", [hex: :poolboy, optional: true]}, {:sbroker, "~> 1.0", [hex: :sbroker, optional: true]}]},
+  "decimal": {:hex, :decimal, "1.3.1", "157b3cedb2bfcb5359372a7766dd7a41091ad34578296e951f58a946fcab49c6", [:mix], []},
+  "earmark": {:hex, :earmark, "1.2.2", "f718159d6b65068e8daeef709ccddae5f7fdc770707d82e7d126f584cd925b74", [:mix], []},
+  "ex_doc": {:hex, :ex_doc, "0.16.1", "b4b8a23602b4ce0e9a5a960a81260d1f7b29635b9652c67e95b0c2f7ccee5e81", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, optional: false]}]}}