Merge branch 'elixir-1.9' into 'develop'

Upgrade to Elixir 1.9

See merge request !2651

.gitlab-ci.yml

-image: elixir:1.8.1
+image: elixir:1.9.4
 
 variables: &global_variables
   POSTGRES_DB: pleroma_test

elixir_buildpack.config

-elixir_version=1.8.2
-erlang_version=21.3.7
+elixir_version=1.9.4
+erlang_version=22.3.4.1

mix.exs

@@ -5,7 +5,7 @@ def project do
     [
       app: :pleroma,
       version: version("2.0.50"),
-      elixir: "~> 1.8",
+      elixir: "~> 1.9",
       elixirc_paths: elixirc_paths(Mix.env()),
       compilers: [:phoenix, :gettext] ++ Mix.compilers(),
       elixirc_options: [warnings_as_errors: warnings_as_errors(Mix.env())],