3 changes to get tmux to work:
update tmux to latest release

remove openssl dependency for libevent (partly because openssl forumla
on chromebrew is broken right now, partly because we dont need it.)

change command line options for wget in crew script:
man page says -N is incompatible with -O (and wget complains every run)
--content-disposition not needed since we specify -O and big files on
github (which are backed by aws) can not be retrieved with this flag,
probably because AWS doesn't support HEAD requests for those files