make it clear @next_time is not set to false

as surfaced in discussion at gh-129
use pending("xxx") in the specs

countering https://travis-ci.org/jmettraux/rufus-scheduler/jobs/32581856
give a bit more spec time for Ruby 2.1.x
drop the conditional pending :-(
Make locks an argument to Scheduler.

Having the lock be an argument makes it easier to implement different
types of locks, since we won't need to subclass from Scheduler and
instead can implement smaller classes.
Prevent running jobs unless a lock is held.

This allows for multiple scheduling processes to run concurrently, but
only one of them will run jobs at any given time.
Add pry development dependency.