Rate::sleep fails to compile on OSX #181

18:20:33 /Users/osrf/jenkins/workspace/ci_osx/ws/install/rclcpp/include/rclcpp/rate.hpp:74:20: error: no viable overloaded '+='
18:20:33     last_interval_ += period_;
18:20:33     ~~~~~~~~~~~~~~ ^  ~~~~~~~
18:20:33 /Users/osrf/jenkins/workspace/ci_osx/ws/src/ros/geometry_experimental/tf2_ros/src/tf2_echo.cpp:142:12: note: in instantiation of member function 'rclcpp::rate::GenericRate<std::__1::chrono::system_clock>::sleep' requested here
18:20:33       rate.sleep();
18:20:33            ^
18:20:33 /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/chrono:782:79: note: candidate function not viable: no known conversion from 'duration<[...], ratio<[...], 1000000000>>' to 'const duration<[...], ratio<[...], 1000000>>' for 1st argument
18:20:33     __attribute__ ((__visibility__("hidden"), __always_inline__)) time_point& operator+=(const duration& __d) {__d_ += __d; return *this;}