Qualify unordered_map when including via tr1

0a1d32a38c3d19e9def19b835c8ea2dbf51aed98 introduced a minor compile error, in that it uses an unqualified `unordered_map` when including it from tr1. This PR adds a `using std::tr1::unordered_map;` when compiling in non-C++11 mode.