Use user data directory for reference docx archive.

This allows the test suite to work without installing pandoc first.
It also brings the docx writer in line with the odt writer.

src/Text/Pandoc/Writers/Docx.hs

@@ -211,7 +211,7 @@ writeDocx opts doc@(Pandoc meta _) = do
   let doc' = walk fixDisplayMath $ doc
   username <- lookup "USERNAME" <$> getEnvironment
   utctime <- getCurrentTime
-  distArchive <- getDefaultReferenceDocx Nothing
+  distArchive <- getDefaultReferenceDocx datadir
   refArchive <- case writerReferenceDocx opts of
                      Just f  -> liftM (toArchive . toLazy) $ B.readFile f
                      Nothing -> getDefaultReferenceDocx datadir