Support for kong 0.12

[sohamkamani]

Currently, the API we use to get targets, does not get targets with weight 0: https://github.com/gojekfarm/ping-kong/blob/master/kong.go#L71

According to kong 0.12, we need to use this API : /upstreams/{name or id}/targets/all/ (https://getkong.org/docs/0.12.x/admin-api/#endpoint)[https://getkong.org/docs/0.12.x/admin-api/#endpoint]

As a consequence of this, once an endpoint is marked as unhealthy, and a weight of 0 is set, it never appears again even if it becomes healthy

[rShetty]

@sohamkamani We can add support for Kong 0.12, but 0.12 kong comes with its own built in health checker. We wanted this library to target mostly lower versions. But definitely adding support for 0.12 is in pipeline too