Raise error if unsupported version (i.e. 7) encountered

[lofidevops]

While ImageMagick 7 is not supported ( #287 ) it would be helpful if wand would explicitly raise an error if version 7 is detected, and recommend installing 6.

Otherwise, you are pointed to http://docs.wand-py.org/en/latest/guide/install.html#install-imagemagick-on-windows which makes it seem like your configuration is incorrect, rather than that the verison isn't supported. (See #287 for comments about wasting time discovering this fact.)