Cope with Ansible >= 2.4 inventory changes

[nphilipp]

This is to deal with changes in Ansible 2.4 which removed ansible.inventory.Inventory, apparently to be replaced by ansible.inventory.manager.InventoryManager.

Here's how it behaved without the changes in this PR:

(ansible-review) nils@gibraltar:~/src/fedora-infra/ansible (master)> ansible-review inventory/builders 
WARN: No configuration file found at /home/nils/.config/ansible-review/config.ini
WARN: Using example standards found at /home/nils/src/ansible-review/lib/ansiblereview/examples
WARN: Using example lint-rules found at /home/nils/src/ansible-review/lib/ansiblereview/examples/lint-rules
Traceback (most recent call last):
  File "/home/nils/src/ansible-review/lib/ansiblereview/inventory.py", line 36, in parse
    ansible.inventory.Inventory(loader=loader, variable_manager=var_manager,
AttributeError: module 'ansible.inventory' has no attribute 'Inventory'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/nils/.virtualenvs/ansible-review/bin/ansible-review", line 11, in <module>
    load_entry_point('ansible-review', 'console_scripts', 'ansible-review')()
  File "/home/nils/src/ansible-review/lib/ansiblereview/__main__.py", line 99, in main
    errors = errors + candidate.review(options, lines)
  File "/home/nils/src/ansible-review/lib/ansiblereview/__init__.py", line 76, in review
    return utils.review(self, settings, lines)
  File "/home/nils/src/ansible-review/lib/ansiblereview/utils/__init__.py", line 120, in review
    result = standard.check(candidate, settings)
  File "/home/nils/src/ansible-review/lib/ansiblereview/inventory.py", line 41, in parse
    result.errors = [Error(None, "Inventory is broken: %s" % e.message)]
AttributeError: 'AttributeError' object has no attribute 'message'
(ansible-review) nils@gibraltar:~/src/fedora-infra/ansible (master)>

This PR also lets ansible-review cope with exceptions caught that don't have a message attribute.

[nphilipp]

NB: I'm not really familiar with Ansible innards myself, so please pay special attention to the actual functional changes in the "Cope with Ansible >= 2.4 inventory changes." commit when reviewing. Thanks.