Feature/bigtop support

fixes COOK-1 https://issues.cask.co/browse/COOK-1
- [x] support for apache bigtop distribution
- [x] ability to specify custom repos without gpg key validation
Does this work?

I get this on CentOS 6.5... which is a float, or maybe a string, but definitely not a single number.
```
  "platform_version": "6.5",
  "platform_family": "rhel",
```

Your yum_repo_url at line 204 will be "6.5" and not "6" so the URL will be incorrect.