[slf4j-dev] [qos-ch/slf4j] 21af69: add travis build cache

[Ceki Gulcu]

  Branch: refs/heads/master
  Home:   https://github.com/qos-ch/slf4j
  Commit: 21af69f3eb5ba657ac8f1437cc7e7870cabcafe1
      https://github.com/qos-ch/slf4j/commit/21af69f3eb5ba657ac8f1437cc7e7870cabcafe1
  Author: Ceki Gulcu <ceki at qos.ch>
  Date:   2021-08-16 (Mon, 16 Aug 2021)

  Changed paths:
    M .travis.yml

  Log Message:
  -----------
  add travis build cache

Signed-off-by: Ceki Gulcu <ceki at qos.ch>