vcloud_attributes response has changed

The response from OrgVdcNetwork#vcloud_attributes has changed in vcloud-core,
as a result of fixing the get_network deprecation in Fog.

See alphagov/vcloud-core#45 for details on the deprecation fix.
Bump vcloud-core version required

We depend on the updated OrgVdcNetwork#vcloud_attributes call
Bump version due to core version bump