Address deprecations of Kernel.<-/2 and Enum.first/1 in elixir v0.12.2

On a related note, the [`Postgrex.Connection.send/2` function](https://github.com/ericmj/postgrex/blob/master/lib/postgrex/connection.ex#L739) now conflicts with elixir's `Kernel.send/2`.

I had to do a quick and dirty hack to postgrex (changing send/2 to send_msg/2) to get Ecto to work, but I think I should leave it to @ericmj to institute the proper fix.