Changes towards Elixir 0.15

Hi!
here are trivial changes to catch up Elixir 0.15.0.

They end with errored test - it is because API of Macro.pipe / unpipe has changed.
I believe `p_expand` and `pipebind` in `Monad.Pipeline` are to be fixed.
It's hard for me to rewrite them by guessing how they worked. Actually I played with them a little bit, but I think my macro-fu is not good enough yet.

Here's a relevant entry in Elixir changelog (0.13.1)

* Backwards incompatible changes
  * [Macro] `Macro.unpipe/1` now returns tuples and `Macro.pipe/2` was removed in favor of `Macro.pipe/3` which explicitly expects the second element of the tuple returned by the new `Macro.unpipe/1`