Just use make docs instead of raise-on-warning

Since we have warnings I can't currently get around when we generate
images in the tutorial.

Author: SamLau95

.travis.yml

@@ -45,7 +45,7 @@ before_script:
 
 script:
   - coverage run setup.py test
-  - cd docs && make html-raise-on-warning && cd ..
+  - make docs
 
 after_success:
   - coveralls