revert tensorflow to 1.5.0 (#514)

lgarithm · web-flow · commit 9693e3d2cf48 · 2018-04-20T23:00:51.000+08:00
see tensorflow/tensorflow#17441 for more details
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -5,4 +5,4 @@ progressbar2>=3.37,<3.38
 scikit-image>=0.13,<0.14
 scipy>=1.0,<1.1
 sphinx>=1.7,<1.8
-tensorflow>=1.7,<1.8
+tensorflow==1.5.0
