https://github.com/jekyll/jekyll/blob/master/docs/_docs/continuous-integration/travis-ci.md
We do not need to use NOKOGIRI_USE_SYSTEM_LIBRARIES to make CI build faster.
If a branch does not have its own cache, Travis CI fetches the default branch cache.
from https://docs.travis-ci.com/user/caching/#fetching-and-storing-caches