GCC 4.6 doesn't provide std::chrono::steady_clock and GCC 4.7 doesn't provide
std::this_thread::sleep_for. I would prefer to support GCC 4.7 but I'm
reverting this since the bots are GCC 4.6.
This reverts commit c5f454957d.
MSVC++ before 2015 Update 2 has a bug in sleep_for where it tries to
implicitly += the input with a nanoseconds variable. Work around this by
using nanoseconds directly (which can be implicitly +='d with
chrono::nanoseconds).
