Add `do` to the list of Elixir keywords

Added the `do` Elixir keyword to the list of Elixir keywords. For example when defining a function in Elixir:

  def sum(a, b) do
    a + b
  end

or when defining a module

  defmodule Mod do
    # stuff
  end
Merge pull request #60 from whatyouhide/patch-1

Add `do` to the list of Elixir keywords