Update to 0.31

Upstream changes:
0.31 2016/08/17
  * Add minimum Perl version to Makefile.PL
  * Match HTTP::Async::Polite version to HTTP::Async (CPANTS)
    Thanks to Mohammad S Anwar (Manwar) for both changes!
  * Replace yahoo.com with metacpan.org in t/real_servers.t

(wen)