remove last dependency

Guillaume Lemaitre · Guillaume Lemaitre · commit 64ff42b9ed6f · 2017-01-01T14:19:15.000+01:00
diff --git a/imblearn/ensemble/balance_cascade.py b/imblearn/ensemble/balance_cascade.py
@@ -9,8 +9,7 @@
 from sklearn.neighbors import KNeighborsClassifier
 from sklearn.utils import check_random_state
 from sklearn.utils.validation import has_fit_parameter
-
-from six import string_types
+from sklearn.externals.six import string_types
 
 from ..base import BaseBinarySampler
 
