Fix link check

Author: vidartf

.travis.yml

@@ -27,7 +27,7 @@ script:
   - py.test
   - npm test
   # Check links in markdown files:
-  - python -m pytest_check_links --link-ext=md -k .md
+  - python -m pytest_check_links -k .md
   # Check docs can be build + links
   - pushd docs
   - make html