Update to 0.72

Upstream changes:
0.72    2018-04-02 19:34:27
        [CHANGES]
         * Expanded docs on Stream's on_read sub return values
         * Overridable timeout for IO::Async::Test::wait_for
         * Hide IO::Async::ChildManager as an internal implementation detail;
           move the docs into main IO::Async::Loop
         * Added $loop->open_process; suggest using that instead of
           ->open_child
         * Begin implementation of $loop->is_running method (part of RT123780)

        [BUGFIXES]
         * Make IO::Async::Future->await wait until it's ready
         * Fix calculation of periodic timer in 'skip' mode (RT124414)
         * Perform synchronous getaddrinfo shortcut even for service-less
           lookups
         * Make sure that errors from underlying read/write functions are
           debug-logged by IO::Async::Stream


(wen)