This reverts commit 96defc5dc2. With PyYAML 4.1 we don't have C extensions on Travis CI: https://github.com/yaml/pyyaml/issues/179 https://github.com/yaml/pyyaml/issues/182 Unfortunately, cython isn't in the APT whitelist either: https://github.com/travis-ci/apt-source-whitelist/issues/37
96defc5dc2
Closes #1073