Add route53 get_checker_ip_ranges method

Gets a list of the IP ranges used by Amazon Route 53 health checkers to check the health of your resources.

http://docs.aws.amazon.com/Route53/latest/APIReference/API_GetCheckerIpRanges.html
These lines are a bit long and could use `assertIn` instead. Maybe something like this is better:

```python
ip_ranges = response['GetCheckerIpRangesResponse']['CheckerIpRanges']
self.assertEqual(len(ip_ranges), 5)
self.assertIn('54.183.255.128/26', ip_ranges)
...
```