Pin vCloud Core to version 0.5.0

Now that vCloud Tools Tester, a development dependency of the majority
of the vCloud Tools gems, depends on vCloud Core version 0.5.0, all gems
must use the same version of vCloud Core to prevent version conflicts.
Merge pull request #91 from gds-operations/73466396-pin-vcloud-core

[#73466396] Pin vCloud Core to version 0.5.0