isdisjoint() can't be use with lists

```
$ ./bin/ansible-lint examples/example.yml
Traceback (most recent call last):
  File "./bin/ansible-lint", line 68, in <module>
    sys.exit(main(sys.argv[1:]))
  File "./bin/ansible-lint", line 61, in main
    matches.extend(rules.run(playbook, tags=set(options.tags), skip_tags=set(options.skip_tags)))
  File "~/Dev/ansible-lint/lib/ansiblelint/__init__.py", line 49, in run
    if rule.tags.isdisjoint(skip_tags):
AttributeError: 'list' object has no attribute 'isdisjoint'
```

```
$ python --version
Python 2.7.2
```