Detect unsupported platforms

Now that we have _some_ JRuby support, I'm trying to detect the
unsupported platforms and try to help the users, if there is something they
can do about it.

@guilleiguaran, @ryandao do you guys have any feedback? I tried not to
over engineer the solution and let it happen require time. If you have other
ideas or want to reword the messages, I'll be quite grateful :)