eager upgrade strategy

otherwise, pip incorrectly determines that pytest has been satisfied even though it hasn't

.travis.yml

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