Check for JRuby and don't try and use `stty`

JRuby doesn't support shelling out to `stty` because the ruby process does
not have the same terminal as the caller (I think). ANyway, JRuby does have a
builtin jline library that can give you all the echoey fancy stuff that you
need here.

Rewinding doesn't work yet, but I don't know how to test it.  I would suggest
simply disabling it but I don't see how because I I can't find source code
for `disable_rewind`, nor does the sample in the README work for me in which
it is called.