Fix multiple runs after multiple watcher invokations

[leoc]

See #43

Given a guard config like this:

guard 'rake', task: 'load', run_on_start: false do
  watch(%r{.+})
end

Before the behavior was:

• First change of watched file, task is executed once
• Second change of watched file, task is executed twice simultaneously
• Third change of watched file, task is executed thrice simultaneously

The expected new behavior is:

Task is executed once, at every change.