revert: restore preferred_cli_env in def project

Author: yordis

mix.exs

@@ -20,14 +20,11 @@ defmodule Tesla.Mixfile do
         plt_add_apps: [:mix, :inets, :idna, :ssl_verify_fun, :ex_unit],
         plt_add_deps: :apps_direct
       ],
-      docs: docs()
+      docs: docs(),
+      preferred_cli_env: [coveralls: :test, "coveralls.html": :test]
     ]
   end
 
-  def cli do
-    [preferred_envs: [coveralls: :test, "coveralls.html": :test]]
-  end
-
   def application do
     [extra_applications: [:logger, :ssl, :inets]]
   end