Fix rbt post for older servers that don't have a diff validation resource.
Fix rbt post for older servers that don't have a diff validation resource.

The addition of validation to rbt post didn't take into account that the diff
validation resource was only introduced in 2.0.x. This change catches the
AttributeError and just passes through to the posting code. Trivial change.

Fixes bug 3635.

beol/rbtools,beol/rbtools,datjwu/rbtools,reviewboard/rbtools,datjwu/rbtools,halvorlu/rbtools,haosdent/rbtools,davidt/rbtools,datjwu/rbtools,haosdent/rbtools,reviewboard/rbtools,haosdent/rbtools,halvorlu/rbtools,halvorlu/rbtools,davidt/rbtools,beol/rbtools,reviewboard/rbtools,davidt/rbtools