Relax dev dep on vcloud-tools-tester

In gds-operations/vcloud-tools-tester#18 we bumped `vcloud-tools-tester`
to version `0.1.2` when resolving a cyclical runtime dependency on
vcloud-core. We now need to relax the development dependency here to
allow vcloud-core to build, at which point we can publish new versions
of the other gems.
Merge pull request #92 from gds-operations/relax-dev-deps

Relax dev dep on vcloud-tools-tester