fix old callable with new builtin callable

I was getting errors on Python 2.7.5 after installing celery.

This fixes those errors by replacing old isinstance(foo, collections.Callable) with callable(foo) in the safe weak function reference module

The test suite still passes for me after this change.

```
  File "python2.7/site-packages/celery/utils/dispatch/saferef.py", line 143, in remove
    if isinstance(function, Callable):
  File "python2.7/abc.py", line 144, in __instancecheck__
    return cls.__subclasscheck__(subtype)
  File "python2.7/abc.py", line 165, in __subclasscheck__
    cls._abc_cache.add(subclass)
  File "python2.7/_weakrefset.py", line 84, in add
    self.data.add(ref(item, self._remove))
Exception TypeError: TypeError('isinstance() arg 2 must be a class, type, or tuple of classes and types',) in <function remove at 0x1033dbcf8> ignored
```