multiple version of the activerdf and adapter gems clash

[Benjamin Heitmann]

when installing new versions of activerdf and the adapters, it is possible
to have e.g. one adapter installed with more then one version simultaniously.

Then the new version is not used, casing the old version to exhibit its bugs, causing great confusion :(

we might need to look for a way to prevent this inside of activerdf:

the minimal solution could be: do more version bumps for the dependencies.
or refuse loading if more the one version of an adapter installed as a gem is detected, thereby forcing the user to uninstall the old gem manually. (better then running into old bugs again)