Remove troublesome ::ratio members

For some reason, MSVC seems to want to instantiate these, even though
nobody ever asks for them (as evidenced by the fact that the builds
passed on other architectures).

example/custom_systems.cpp

@@ -103,7 +103,7 @@ template<Unit U>
 std::ostream& operator<<(std::ostream& os, const U& u)
 {
   using unit_type = std::remove_cvref_t<decltype(u)>;
-  return os << unit_type::ratio << " x " << unit_type::reference::symbol.standard();
+  return os << as_ratio(unit_type::mag) << " x " << unit_type::reference::symbol.standard();
 }
 
 void what_is_your_ratio()