Increase Tensorflow min version to 1.14 (#320)

requirements.txt

@@ -4,7 +4,7 @@ torchvision==0.3.0
 scipy>=1.3.0
 gitpython==2.1.11
 torchnet==0.0.4
-tensorflow>=1.13
+tensorflow>=1.14
 pydot==1.4.1
 tabulate==0.8.3
 pandas>=0.22.0