travis build fix: libmemcahced changed build-aux/missing to return 0, as it did previously

so that it can compile on travis. although i tried many things in order not to have to do this hack (rerunning autorecon -vfi && make dist) etc, but for some reason make always wants to rerun aclocal, which it shouldn't do. but i don't know enough about automake to say why, it probably wants raising as an upstream bug.