Changed binary_to_(integer/float) into String.to_(integer/float).

[kexoth]

It was refactored as you can see elixir-lang/elixir#1044

Now it can be used either as :erlang.binary_to_integer/1 or as String.to_integer/1 which should be the Elixir way, according to this refactoring 😄

Thanks for this great book! 👍