update tensorflow api to 1.8

Author: xu-song

modules.py

@@ -224,7 +224,7 @@ def multihead_attention(queries,
         # Causality = Future blinding
         if causality:
             diag_vals = tf.ones_like(outputs[0, :, :]) # (T_q, T_k)
-            tril = tf.contrib.linalg.LinearOperatorTriL(diag_vals).to_dense() # (T_q, T_k)
+            tril = tf.linalg.LinearOperatorLowerTriangular(diag_vals).to_dense() # (T_q, T_k)
             masks = tf.tile(tf.expand_dims(tril, 0), [tf.shape(outputs)[0], 1, 1]) # (h*N, T_q, T_k)
 
             paddings = tf.ones_like(masks)*(-2**32+1)