Fix Traits.h std::* declarations conflict with libc++

When compiling folly with clang and libc++, the declaration in folly/Traits.h
clash with the ones from the system library. Fix this by including the original
ones, instead of trying to do that ourselves.
Merge pull request #1 from mcuelenaere/fix_std_declarations_conflicts_osx

Fix Traits.h std::* declarations conflict with libc++