Update .travis.yml

Author: lvapeab

.travis.yml

@@ -154,8 +154,9 @@ install:
   # set library path
   - export LD_LIBRARY_PATH=$HOME/miniconda/envs/test-environment/lib/:$LD_LIBRARY_PATH
 
-  # install TensorFlow (CPU version).
-  - pip install tensorflow
+  # install 
+  orFlow (CPU version).
+  - pip install tensorflow==1.14.0
 
     # detect whether core files are changed or not (from Keras)
   - export CORE_CHANGED=False;