Support out of sync version for upgradeable contracts

[frangio]

Sometimes the versioning for @openzeppelin/contracts and @openzeppelin/contracts-upgradeable gets out of sync because we need to release fixes specific for the upgradeable contracts. This is not supported in Wizard currently and we were not able to merge #96 as a result. The specific problem is that the version included in the source code when opening in Remix is taken from the non-upgradeable contracts. We need to be aware of both versions and choose accordingly.