A ReviewBoardServer's rb_version attribute is set via its check_api_version()
method. This is fine, but our __init__ didn't set a default (dangerous) and if
the check_api_version() lookup failed then the attribute simply didn't exist
(broken). This caused stacktraces when the lookup fails, a user reports that
this happens when his clock is out of sync causing authentication issues...

Traceback (most recent call last):
File "/share/rbtools/rbtools/postreview.py", line 4114, in <module>
  main()
File "/share/rbtools/rbtools/postreview.py", line 4081, in main
  if (parse_version(server.rb_version) >= parse_version('1.5.2') and
AttributeError: 'ReviewBoardServer' object has no attribute 'rb_version'

Adding a default parameter so check_api_version() failures will simply result
in version checks reporting that we're ancient. Also fixed a version check
which was doing string comparison rather than comparing versions.

This is available in the following branch of my repo...
https://github.com/atagar/rbtools/tree/attr_defaulting

Tested by disabling my authentication credentials and running post-review for a
perforce repository.

We've been using this change against RBTools 0.3. I've adapted the change for
the current master but this copy of it hasn't been exercised.