Made the driver extend from Mink 1.5 CoreDriver

@everzet currently, the driver is incompatible with Mink dev-develop as ``~1.4.3`` means ``>=1.4.3,<1.5-dev`` (the ``~`` operator allows bumping only the last part of the specified version number).
With my change, using the develop branch becomes required (note that the ``@dev`` flag is here to allow running the testsuite while Mink 1.5 is not released yet).

So you should tag a version of the driver before merging this.