fix: Travis build (#43)

.travis.yml

@@ -1,10 +1,10 @@
 language: python
 python:
-#  - "2.7"
   - "3.6"
 # command to install dependencies
 install:
   - pip install -r requirements.txt
+  - pip install -r extra-requirements.txt
   - pip install -r test-requirements.txt
 # command to run tests
 script: