classad::Value::IsNumber() now considers booleans as numbers. #3468

Boolean values can be used in arthimetic operations in a classad
expression, and it's common for preemption rank expressions in Condor to
evaluate to a boolean. In fact, use of Value::IsNumber() in the
negotiator was causing the default NEGOTIATOR_PRE_JOB_RANK to not sort
properly.
Version history for boolean values in RANK expressions. #3468