fix JRuby specs

* avoid attr_accessor, which RSpec doesn't like stubbing on JRuby
* prevent specs from using :ronn group, since it's not available on JRuby
* fix Pry spec for new Pry version
add missing require
make guard synchronous

  * Guard.start + Guard.stop basically form a "main" loop

  * no "preserved state"

  * "no interactor" means interactor in interruptible sleep() mode

  * give Pry some time to start

  * use Queue to respond to changes in main Thread

  * handle signals asynchronously
dump Guard:guard + fix specs (stub verification)