Note hard dep on newer python-idb more clearly

[Llammissar]

At this point, pip install python-idb installs version 0.5.0, which does not have idautils.Names(), causing conversion to fail on the missing symbol. git blame tells me the current minimum version is probably python-idb@dd1a7c76, which is when that whole function was added.

So I'd like to see it call out the fact that a newer version is not just a rec, but a req. (Also, might be prudent to put a --user after the command in the suggestion to use setup.py.)