Install libreadline on buildbot VM images, check for libjemalloc1

Starting from 5.5.40, readline detection has been fixed, and system readline is used. It brings a new dependency at least in debian packaging. Tests pull libreadline5 from default repos when they can, and fail when they can't.

Instead, it should be pre-installed on VM images.

Currently at least vm-wheezy-*-install.qcow2 and vm-precise-*-install.qcow2 are known to have a problem due to the missing library.

And while doing this, please also check that libjemalloc1 has been installed (and install if it's not), it's another new dependency.