remove upgrade from test_requirements

this is not needed as we set specific versions

.travis.yml

@@ -33,7 +33,7 @@ python:
 install:
     - pip install --upgrade pip
     - pip install -r requirements.txt
-    - pip install -r test_requirements.txt --upgrade --upgrade-strategy only-if-needed
+    - pip install -r test_requirements.txt
     - pip install -r docs_requirements.txt
     - pip install .