update requirements.txt to include numy version restrictions

.travis.yml

@@ -7,6 +7,7 @@ before_install:
   - pip install -U pip
   - pip install -U pytest
   - pip install codecov
+  - pip install catboost
 install:
   - pip install -r requirements.txt
 script: pytest # run tests

requirements.txt

 pandas
-numpy
+numpy>=1.17
 seaborn
 matplotlib
 IPython