Fixed to comply with latest Elixir syntax

Elixir 0.11.2 seemes to tweak anonymous capture syntax a little bit. (ellihandler is not the only project to be affected: https://github.com/azukiapp/elixir-protobuf/issues/2; https://github.com/elixir-lang/elixir/issues/1820)
This little patch aimed to fix those issues.