From #4039: Fix test by pinning attrs to 17.4+

Author: arradha

.travis.yml

@@ -46,6 +46,7 @@ before_install:
         tar -xzf geckodriver-v0.19.1-linux64.tar.gz -C geckodriver
         export PATH=$PATH:$PWD/geckodriver
       fi
+    - pip install "attrs>=17.4.0"
 
 install:
     - pip install --pre .[test]