Update to Elixir 0.11.x

This PR will update HTTPoison to use the latest stable version of Elixir. It also removes the `mix.lock` from the project since HTTPoison is a library and not an application.

Here is [an excellent blog post](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/) from Yehuda Katz on lockfile usage specific to Bundler/Rubygems.