Update .travis.yml

.travis.yml

@@ -27,7 +27,7 @@ install:
     - pip install pylint
     - pip install python-coveralls
     - pip install coverage==4.5.4
-    - pip install -r requirements.txt
+    - pip install --no-use-pep517 -r requirements.txt
     - pip install .
 # command to run tests
 script: