update tensorflow version to 1.8.0; fix reduce mean convert

docs/installation/env_requirement.rst

@@ -53,7 +53,7 @@ Optional dependencies
       - apt-get install android-tools-adb
       - Required by Android run, >= 1.0.32
     * - TensorFlow
-      - pip install -I tensorflow==1.6.0
+      - pip install -I tensorflow==1.8.0
       - Required by TensorFlow model
     * - Docker
       - `docker installation guide <https://docs.docker.com/install/linux/docker-ce/ubuntu/#set-up-the-repository>`__

docs/installation/manual_setup.rst

@@ -65,7 +65,7 @@ Install TensorFlow (Optional)
 
 .. code:: sh
 
-    pip install -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com tensorflow==1.6.0
+    pip install -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com tensorflow==1.8.0
 
 
 Install Caffe (Optional)

mace/python/tools/converter_tool/tensorflow_converter.py

@@ -806,7 +806,7 @@ class TensorflowConverter(base_converter.ConverterInterface):
 
         reduce_type_arg = op.arg.add()
         reduce_type_arg.name = MaceKeyword.mace_reduce_type_str
-        reduce_type_arg.i = ReduceType.MEAN
+        reduce_type_arg.i = ReduceType.MEAN.value
 
         axis_arg = op.arg.add()
         axis_arg.name = MaceKeyword.mace_axis_str