Update .travis.yml

Pin requirements here, too.

Author: mchladek

.travis.yml

@@ -18,19 +18,19 @@ matrix:
 sudo: false
 
 install:
-  - pip install numpy>=1.13
-  - pip install nltk
+  - pip install numpy==1.14
+  - pip install nltk==3.2.5
   - pip install scikit-learn==0.19.0
-  - pip install pandas
-  - pip install scipy
-  - pip install tensorflow>=1.4
-  - pip install h5py
-  - pip install keras
-  - pip install shapely
-  - pip install pytest
-  - pip install pytest-cov
-  - pip install requests
-  - pip install google-cloud-language
+  - pip install pandas==0.22.0
+  - pip install scipy==1.0.0
+  - pip install tensorflow==1.5
+  - pip install h5py==2.7.1
+  - pip install keras==2.1.4
+  - pip install shapely==1.6.4.post2
+  - pip install pytest==6.2.2
+  - pip install pytest-cov==2.11.1
+  - pip install requests==2.18.4
+  - pip install google-cloud-language==1.3.0
   - |
     if [[ $FLAKE8 ]]; then
       pip install flake8