Replace using with typedef for compatibility with gcc-4.6
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index 522d8d0..f25a8b6 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -309,7 +309,7 @@
   using char_type = Char;
 
   template <typename T>
-  using formatter_type = printf_formatter<T>;
+  struct formatter_type { typedef printf_formatter<T> type; };
 
  private:
   using base = internal::context_base<OutputIt, basic_printf_context, Char>;