BF: allow for ValueError setting long into int64

See discussion at
https://github.com/nipy/nibabel/pull/191#issuecomment-23382942

Numpy 1.4.1 (maybe others) gives a confusing ValueError for something
like this::

   arr = np.zeros(2, dtype=np.int64)
   arr[0:1] = 2**64 # Python long

Add ValueError as another caught exception type for this case.