remove version pin

Author: willingc

.travis.yml

@@ -26,6 +26,7 @@ before_install:
 install:
 # Don't let requirements pull in tornado 5 yet except for jupyterhub master
     - if [ $JHUB_VER != "master" -a $JHUB_VER != "0.9.1" ]; then pip install "tornado<5.0"; fi
+    - pip install attrs>17.4.0
     - pip install --pre -r jupyterhub/dev-requirements.txt
     - pip install --pre -e jupyterhub