travis updated for windows

Author: tawabshakeel

.travis.yml

@@ -19,8 +19,8 @@ jobs:
       env: PATH=/c/Python36:/c/Python36/Scripts:$PATH
 install:
   - pip3 install --upgrade pip  # all three OSes agree about 'pip3'
-  - pip install -r requirements.txt
-  - pip install explainx
+  - pip3 install -r requirements.txt
+  - pip3 install explainx
 
 # 'python' points to Python 2.7 on macOS but points to Python 3.8 on Linux and Windows
 # 'python3' is a 'command not found' error on Windows but 'py' works on Windows only