always use <typetraits> before <tr1/typetraits>.

fixes a problem with gcc-4.3, where <tr1/typetraits> and <typetraits> are present:
/usr/people/nolte/numerics/dune/dune-common/dune/common/typetraits.hh:274: error: ‘std::enable_if’ has not been declared

[[Imported from SVN: r5453]]