Revert "Upgrade Elixir to 1.6.6 (#2653)"

This reverts commit d3a9fbf62f.
rebar2 (which is still used) is not able to cope with this change
it should check lib/elixir/src/elixir.app.src instead of src/elixir.app.src
as src/elixir.app.src moved away, currently check fails and this breaks
jenkins tests.

options: use rebar3, or drop rebar completely to rely only on mix.

mix.exs

@@ -5,7 +5,7 @@ defmodule Ejabberd.Mixfile do
     [app: :ejabberd,
      version: "18.9.0",
      description: description(),
-     elixir: "~> 1.6",
+     elixir: "~> 1.4",
      elixirc_paths: ["lib"],
      compile_path: ".",
      compilers: [:asn1] ++ Mix.compilers,

rebar.config

@@ -47,7 +47,7 @@
                                            {tag, {if_version_above, "19", "develop", "2.5.3"}}}}},
         %% Elixir support, needed to run tests
         {if_var_true, elixir, {elixir, ".*", {git, "https://github.com/elixir-lang/elixir",
-                                              {tag, "v1.6.6"}}}},
+                                              {tag, {if_version_above, "17", "v1.4.4", "v1.1.1"}}}}},
         %% TODO: When modules are fully migrated to new structure and mix, we will not need anymore rebar_elixir_plugin
         {if_not_rebar3, {if_var_true, elixir, {rebar_elixir_plugin, ".*",
                                                {git, "https://github.com/processone/rebar_elixir_plugin", "0.1.0"}}}},