Update ext-libevent version installed for build runs on Travis CI.
Update ext-libevent version installed for build runs on Travis CI.

kaduev13/event-loop,steverhoades/event-loop,mkrauser/event-loop,clue-labs/event-loop,mkrauser/event-loop,reactphp/event-loop,kaduev13/event-loop,steverhoades/event-loop