Include libreadline5 and libreadline5-dev

The CF PHP buildpack we're currently building requires libreadline.so.5,
which is part of the libreadline-dev C++ library, version 5.

Prior to this commit, the rootfs only performed an apt-get of
libreadline6-dev.  These packages are not compatible side-by-side, so
libreadline6-dev has been replaced with libreadline6.