Update travis settings for release-branches

- drop the deprecated flag `--deployment` of bundle (so we have to specify the bundler caching path)

.travis.yml

@@ -4,19 +4,29 @@ dist: bionic
 language: ruby
 rvm: 2.7.0
 
-cache: bundler
+cache:
+  directories:
+    - $TRAVIS_BUILD_DIR/vendor/bundle
+
+before_install:
+  - gem install bundler:2.2.4   # match the Gemfile.lock, travis' bundle is 2.1.2
+  - bundle config path 'vendor/bundle'
+
+install:
+  # overriding to drop the travis `--development` flag
+  - bundle install --jobs=3 --retry=3
 
 addons:
   apt:
     packages:
-      - libcurl4-openssl-dev # required to avoid SSL error (for htmlproofer)
+      - libcurl4-openssl-dev  # required to avoid SSL error (for htmlproofer)
 
 script:
   - git -C "$HOME" clone "$BUILDER_REPO" --depth=1 -q
   - eval "$BUILD_CMD"
 
 branches:
-  only: production
+  only: /.*-stable$/
 
 git:
   depth: false # for posts lastmod