Travis failures:

valgrind: m_scheduler/sema.c:104 (vgModuleLocal_sema_down): Assertion 'sema->owner_lwpid != lwpid' failed.
host stacktrace:
==1296==    at 0x38083F48: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x38084064: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380841F1: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x38135DAE: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380D328D: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x3809A4AC: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x3809AE43: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380988CF: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
sched status:
  running_tid=0
Thread 1: status = VgTs_WaitSys (lwpid 1296)
==1296==    at 0x5729730: __poll_nocancel (syscall-template.S:84)
==1296==    by 0x4348DF: daemon_poll (daemon.c:78)
==1296==    by 0x4169E7: io_poll_lightningd (lightningd.c:543)
==1296==    by 0x471ECD: io_loop (poll.c:282)
==1296==    by 0x416E06: main (lightningd.c:744)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>