Handle `any-ipv4` address as `any`

```
ERROR OUTPUT:
Traceback (most recent call last):
  File "/opt/fwunit/releng/bin/fwunit", line 9, in <module>
    load_entry_point('fwunit==0.1.0', 'console_scripts', 'fwunit')()
  File "/opt/fwunit/releng/src/fwunit/fwunit/scripts.py", line 87, in main
    rules = ep(src_cfg, cfg)
  File "/opt/fwunit/releng/src/fwunit/fwunit/srx/scripts.py", line 16, in run
    return policies_to_rules(app_map, firewall)
  File "/opt/fwunit/releng/src/fwunit/fwunit/srx/process.py", line 24, in policies_to_rules
    firewall.zones, policies_by_zone_pair)
  File "/opt/fwunit/releng/src/fwunit/fwunit/srx/process.py", line 127, in process_address_sets_per_policy
    (get_from(a) for a in pol.source_addresses), IPSet())
  File "/opt/fwunit/releng/src/fwunit/fwunit/srx/process.py", line 127, in <genexpr>
    (get_from(a) for a in pol.source_addresses), IPSet())
  File "/opt/fwunit/releng/src/fwunit/fwunit/srx/process.py", line 122, in <lambda>
    get_from = lambda a: a if isinstance(a, IPSet) else from_addrbook[a]
KeyError: 'any-ipv4'
```