Fix #59; notify when transitioning from green to red

This makes an extra request to the data store for the threshold.
Fix spec
Bump up maximum class length

I thought I could get it back under 100, but that doesn't seem
likely. Instead of changing it every time I change Stoplight::Light,
I'll just give us some breathing room.
Return the threshold from sync and cache it

This way the stoplight doesn't have to make an extra request to the
data store when handling an error.
Pass the failure to the notifier

Although the default notifier wasn't showing error information,
it's something we were doing with our custom notifier. Getting the
failure from the data store didn't make a lot of sense because it
could be gone by the time we want to notify about it.
Update changelog