Travis + Jekyll + ruby: cache and bundle version problem


I am using for the first time Travis … be indulgent.

I cannot generate a build without errors: the job log displays “could not download cache” and "find_spec_for_exe: can’t find gem bundler (>= 0.a) with executable bundle (Gem::GemNotFoundException)
The command “eval bundle install --jobs=3 --retry=3 --deployment --path=${BUNDLE_PATH:-vendor/bundle} " failed. Retrying, 2 of 3.”

my .travis.yml:

language: ruby
  - 2.5.3                          <<<<< I added this
cache: bundler
os: linux
dist: xenial

  - bundle exec jekyll build

  - chmod 777
  - ./

Try updating the gemfile.lock that mentions the bundler version

Gemfile.lock has : BUNDLED WITH 2.1.4
On my machine:
Ruby version: 2.5.5p157
gem --version 3.1.4

Ruby version in Travis log: 2.5.3p105
rvm 1.29.7 in Travis log
gem version 2.7.7 in Travis log

2.1.4 is the latest version for Bundler ! what can I update ?
I excluded vendor in _config.yml as notified by Jekyll.