Make libffi a berkshelf dependency since we're always using 3.x.x now.
Add gdbm as a dependency to ruby on all platforms.
Remove -R flag from LDFLAGS to make Mac OSX compiler happy.
Move ffi libraries correctly on centos.
Force ruby to gdbm in order not to pick up dbm if it exists on the system.
Merge pull request #147 from opscode/libffi-health-check

chefdk CI fixes