travis: Don't use a local jemalloc

Turns out they don't have the `je_` prefix, so we can't use the system-installed
jemalloc.
auto merge of #14858 : alexcrichton/rust/no-jemalloc-on-travis, r=thestinger

Turns out they don't have the `je_` prefix, so we can't use the system-installed
jemalloc.