remove pytest-warnings

maybe this is what's pinning pytest to 3.3

Author: minrk

.travis.yml

@@ -9,7 +9,8 @@ python:
 sudo: false
 install:
   - pip install --upgrade setuptools pip
-  - pip install --upgrade --pre -e .[test] pytest-cov pytest-warnings codecov 'coverage<5'
+  - pip install --upgrade --pre -e .[test] pytest-cov codecov 'coverage<5'
+  - pip freeze
 script:
   - py.test --cov jupyter_client jupyter_client
 after_success: