Adjust to some elixir changes

lib/geef/blob.ex

@@ -1,6 +1,6 @@
 defmodule Geef.Blob do
   alias Geef.Object
-  import :macros, Object
+  import Object, only: :macros
 
   def lookup(repo, id) do
     case :geef_obj.lookup(repo, id) do

lib/geef/commit.ex

@@ -1,6 +1,6 @@
 defmodule Geef.Commit do
   alias Geef.Object
-  import :macros, Object
+  import Object, only: :macros
 
   def lookup(repo, id) do
     case :geef_commit.lookup(repo, id) do

lib/geef/tag.ex

@@ -1,6 +1,6 @@
 defmodule Geef.Tag do
   alias Geef.Object
-  import :macros, Object
+  import Object, only: :macros
   import Geef
 
   def peel(tag = Object[type: :tag]) do

lib/geef/tree.ex

@@ -9,7 +9,7 @@ defrecord Geef.Tree, Record.extract(:geef_object, from: "src/geef_records.hrl")
   alias Geef.Object
   alias Geef.TreeEntry
 
-  import :macros, Geef.Object
+  import Object, only: :macros
 
   def lookup(repo, id) do
     case :geef_tree.lookup(repo, id) do