manually backport pr #421

set pip upgrade strategy to eager

Author: minrk

.travis.yml

@@ -1,7 +1,6 @@
 language: python
 python:
-    - "nightly"
-    - '3.6-dev'
+    - 3.6
     - 3.5
     - 3.4
     - 3.3
@@ -15,7 +14,8 @@ install:
       pip install pip==10.*
     fi
   - pip install --upgrade setuptools pip
-  - pip install --upgrade --pre -e .[test] pytest-cov pytest-warnings codecov
+  - pip install --upgrade --upgrade-strategy eager --pre -e .[test] pytest-cov pytest-warnings codecov
+  - pip freeze
 script:
   - py.test --cov jupyter_client jupyter_client
 after_success: