Update README with optional args to canaries
Construct InstancePinger with app url and instance count values

* Clean up self-stubbing in InstancesAviary spec and convert to allow/expect
* Make ruby more idiomatic

[#67166568]
Add methods to InstancePinger to make it check-able

* The main 'check' function requires its target to
have #ok? and #error_message methods.

[#67166568]
Pass separate zero-downtime and instance-canary app counts to aviary app.

[#67166568]
Add /instances_pinged_aviary endpoint to aviary canary

[#67166568]
Ignore non-200 responses in instance pinger

* Switch to using get_response to get HTTP code along with body
Add ability to run canaries without installing gem