Install rubygems 2.7.8 and bundler ~> 1.17 on Travis

Author: krzysiek1507

.travis.yml

@@ -6,9 +6,9 @@ cache:
     - vendor/bundle
 
 before_install:
-  - "travis_retry gem update --system"
-  - "travis_retry gem update bundler"
-install: bundle install --path=vendor/bundle --retry=3 --jobs=3
+  - "travis_retry gem update --system 2.7.8"
+  - "travis_retry gem install bundler -v '1.17.3'"
+install: BUNDLER_VERSION=1.17.3 bundle install --path=vendor/bundle --retry=3 --jobs=3
 
 script:
   - bundle exec rake ci