Relax Elixir version requirement

Currently, ExIrc specifies its Elixir version to 1.0.0. This will result in a warning message when compiling on anything newer than 1.0.x, e.g. the latest elixir master, which is currently 1.1-dev. Relaxing this version requirement will remove this warning message, and since Elixir follows Semantic Versioning, this should not be an issue.