Swap these error names

Author: gjtorikian

test/html/pipeline/require_helper_test.rb

@@ -13,14 +13,14 @@ def test_raises_mising_dependency_error
   end
 
   def test_raises_error_including_message
-    error = assert_raises(LoadError) do
+    error = assert_raises(MissingDependencyError) do
       HTML::Pipeline.require_dependency('non-existant', 'SomeClass')
     end
     assert_includes(error.message, "Missing dependency 'non-existant' for SomeClass. See README.md for details.")
   end
 
   def test_raises_error_includes_underlying_message
-    error = assert_raises LoadError do
+    error = assert_raises MissingDependencyError do
       HTML::Pipeline.require_dependency('non-existant', 'SomeClass')
     end
     assert_includes(error.message, 'LoadError: cannot load such file')