`logsumexp(all_log_proba)` does not work because `all_log_proba` is list.

```
======================================================================
ERROR: Predict probabilities.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/gilles/Sources/scikit-learn/sklearn/ensemble/tests/test_bagging.py", line 163, in test_probability
    np.exp(ensemble.predict_log_proba(X_test)))
  File "/home/gilles/Sources/scikit-learn/sklearn/ensemble/bagging.py", line 629, in predict_log_proba
    log_proba = logsumexp(all_log_proba) - np.log(self.n_estimators)
  File "/home/gilles/Sources/scikit-learn/sklearn/utils/extmath.py", line 315, in logsumexp
    arr = np.rollaxis(arr, axis)
  File "/usr/local/lib/python2.7/dist-packages/numpy-1.6.2-py2.7-linux-i686.egg/numpy/core/numeric.py", line 1105, in rollaxis
    n = a.ndim
AttributeError: 'list' object has no attribute 'ndim'
```