Fixes the reported line for failures using assert_receive

It needed to be turned into a macro and some tinkering with
external vars was needed.

lib/blank_assertions.ex

@@ -1,6 +1,4 @@
 defmodule BlankAssertions do
-  require ExUnit.Assertions
-
   defmacro assert(expr) do
     if contains_blank?(expr) do
       code = Macro.escape(expr)
@@ -27,14 +25,15 @@ defmodule BlankAssertions do
     end
   end
 
-  def assert_receive(expr) do
-    if expr == :__ do
-      raise ExUnit.AssertionError, expr: expr
+  defmacro assert_receive(expr) do
+    code = Macro.escape(expr)
+    if contains_blank?(expr) do
+      quote do
+        raise ExUnit.AssertionError, expr: {:assert_receive, [], [unquote(code)]}
+      end
     else
-      receive do
-        ^expr -> true
-      after
-        100 -> flunk("No message matching #{expr} found in mailbox")
+      quote do
+        ExUnit.Assertions.assert_receive(var!(expr), 100)
       end
     end
   end

lib/koans.ex

@@ -41,13 +41,14 @@ defmodule Koans do
   end
 
   defp create_vars(amount) do
-    Enum.map(0..amount, fn (idx) -> quote do: elem(converted, unquote(idx)) end)
+    for id <- 0..amount, do: quote do: elem(converted, unquote(id))
   end
 
   defmacro __using__(_opts) do
     quote do
       @compile :nowarn_unused_vars
       Module.register_attribute(__MODULE__, :koans, accumulate: true)
+
       require ExUnit.Assertions
       import Koans
       import BlankAssertions