Caching CI Build Assets

[s0kil]

On every commit to the gh-pages branch, the CI processes every logo, this is highly inefficient and time-consuming. Suggesting to use Travis CI caching capabilities to save the optimized logo assets and reuse on build.

References:

• https://travis-ci.community/t/cache-a-single-file/132
• Travis Cache directory does not work for a folder travis-ci/travis-ci#9814