chefdk CI fixes

Ended up fixing 4 things:

* Make `libffi` a dependency of berks. `version.to_f` logic is not applicable anymore since we're using git references for version now. 
* Remove `-R` from Mac flags
* Relocate libffi libraries correctly on centos
* Make `gdbm` a dependency of ruby otherwise on ubuntu ruby is linked ith system `gdbm`.