Avoid including VersionSorter as mixin

Tests should use the module as it's meant to be used in the wild.

test/version_sorter_test.rb

@@ -2,18 +2,16 @@
 require 'version_sorter'
 
 class VersionSorterTest < Test::Unit::TestCase
-  include VersionSorter
-
   def test_sorts_versions_correctly
     versions = %w(1.0.9 1.0.10 2.0 3.1.4.2 1.0.9a)
     sorted_versions = %w( 1.0.9 1.0.9a 1.0.10 2.0 3.1.4.2 )
 
-    assert_equal sorted_versions, sort(versions)
+    assert_equal sorted_versions, VersionSorter.sort(versions)
   end
 
   def test_returns_same_object
     versions = %w( 2.0 1.0 0.5 )
-    sorted = sort(versions)
+    sorted = VersionSorter.sort(versions)
 
     assert_equal versions[2].object_id, sorted[0].object_id
   end
@@ -22,6 +20,6 @@ def test_reverse_sorts_versions_correctly
     versions = %w(1.0.9 1.0.10 2.0 3.1.4.2 1.0.9a)
     sorted_versions = %w( 3.1.4.2 2.0 1.0.10 1.0.9a 1.0.9 )
 
-    assert_equal sorted_versions, rsort(versions)
+    assert_equal sorted_versions, VersionSorter.rsort(versions)
   end
 end