install tf 1.14 for travis

.travis.yml

@@ -10,7 +10,7 @@ install:
   - pip install . --no-cache-dir
 script:
   - pytest --cov=pwlf -p no:warnings tests/tests.py
-  - pip install "tensorflow>=1.0.0,<2.0.0"
+  - pip install "tensorflow==1.14.0"
   - pytest --cov=pwlf --cov-append -p no:warnings tests/teststf.py
 before_script:
   - flake8 pwlf