bin/prepare: Use vendored rpm-4.11.1 on el5

CentOS 5 can't use the yum-printbuilddep Python script because
'rpm.spec' doesn't exist. So instead use a prebuilt rpm-4.11.1 that
has a prefix of /app/.heroku/vendor. This version of rpm provides the
rpmspec binary, which can be used to easily print buildrequires info.