Use @callback instead of use Behaviour (#40)

lib/porcelain/drivers/driver_common.ex

@@ -1,17 +1,15 @@
 defmodule Porcelain.Driver.Common do
   @moduledoc false
 
-  use Behaviour
+  @callback exec(prog :: binary, args :: [binary], opts :: Keyword.t) :: any
+  @callback exec_shell(prog :: binary, opts :: Keyword.t) :: any
+  @callback spawn(prog :: binary, args :: [binary], opts :: Keyword.t) :: Porcelain.Process
+  @callback spawn_shell(prog :: binary, opts :: Keyword.t) :: Porcelain.Process
 
-  defcallback exec(prog :: binary, args :: [binary], opts :: Keyword.t)
-  defcallback exec_shell(prog :: binary, opts :: Keyword.t)
-  defcallback spawn(prog :: binary, args :: [binary], opts :: Keyword.t)
-  defcallback spawn_shell(prog :: binary, opts :: Keyword.t)
-
-  defcallback feed_input(port :: port, input :: iodata)
-  defcallback process_data(data :: binary, output :: any, error :: any)
-  defcallback send_signal(port :: port, signal :: Porcelain.Process.signal)
-  defcallback stop_process(port :: port) :: integer
+  @callback feed_input(port :: port, input :: iodata) :: any
+  @callback process_data(data :: binary, output :: any, error :: any) :: tuple
+  @callback send_signal(port :: port, signal :: Porcelain.Process.signal) :: boolean | nil
+  @callback stop_process(port :: port) :: integer
 
 
   alias Porcelain.Driver.Common.StreamServer
@@ -162,7 +160,7 @@ defmodule Porcelain.Driver.Common do
   end
 
   defp send_result(out, err, opt, result) do
-    result = if opt == :discard, do: result = nil, else: result
+    result = if opt == :discard, do: nil, else: result
     msg = {self(), :result, result}
 
     out_ret = case out do

mix.lock

@@ -1,2 +1,2 @@
-%{"earmark": {:hex, :earmark, "1.0.1", "2c2cd903bfdc3de3f189bd9a8d4569a075b88a8981ded9a0d95672f6e2b63141", [:mix], []},
-  "ex_doc": {:hex, :ex_doc, "0.13.0", "aa2f8fe4c6136a2f7cfc0a7e06805f82530e91df00e2bff4b4362002b43ada65", [:mix], [{:earmark, "~> 1.0", [hex: :earmark, optional: false]}]}}
+%{"earmark": {:hex, :earmark, "1.0.1", "2c2cd903bfdc3de3f189bd9a8d4569a075b88a8981ded9a0d95672f6e2b63141", [:mix], [], "hexpm"},
+  "ex_doc": {:hex, :ex_doc, "0.13.0", "aa2f8fe4c6136a2f7cfc0a7e06805f82530e91df00e2bff4b4362002b43ada65", [:mix], [{:earmark, "~> 1.0", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"}}