Changes to_string to Macro.to_string

Fix for this error:

** (CompileError) ... /deps/httpotion/lib/httpotion.ex:123: function to_string/1 undefined
    lists.erl:1323: :lists.foreach/2
    /private/tmp/elixir-8WDo/elixir-0.10.1/lib/elixir/lib/kernel/parallel_compiler.ex:92: Kernel.ParallelCompiler."-spawn_compilers/8-fun-0-"/3