replace keep_dims (deprecated in TF) with keepdims

Exact warning message:
```
From align/detect_face.py:212: calling reduce_sum (from tensorflow.python.ops.math_ops) with keep_dims is deprecated and will be removed in a future version.
Instructions for updating:
keep_dims is deprecated, use keepdims instead
```

src/align/detect_face.py

@@ -207,9 +207,9 @@ def fc(self, inp, num_out, name, relu=True):
     """
     @layer
     def softmax(self, target, axis, name=None):
-        max_axis = tf.reduce_max(target, axis, keep_dims=True)
+        max_axis = tf.reduce_max(target, axis, keepdims=True)
         target_exp = tf.exp(target-max_axis)
-        normalize = tf.reduce_sum(target_exp, axis, keep_dims=True)
+        normalize = tf.reduce_sum(target_exp, axis, keepdims=True)
         softmax = tf.div(target_exp, normalize, name)
         return softmax