Migrated to v2 of the REST API

[lfepp]

PagerDuty has deprecated v1 of the REST API so I have migrated your code to v2. In addition, I made a few other changes to the code:

1. Updated code to be within the RuboCop style guidelines
2. Updated the code to handle users with multiple email and/or phone contact methods
3. Updated the second command line argument to be a service over a schedule. I think this is more useful as it will list the currently on-call user for the service regardless of whether the escalation policy tied to the service has a schedule on level one or just has the user added as level one. If you disagree I'm happy to close the PR and submit another based upon a schedule ID argument.

I ran a few tests on my end but please feel free to run a few tests as well and plug this into IRC before merging!