fix: travis bundler issue

Author: rashidsp

.travis.yml

@@ -2,11 +2,14 @@ branch:
   only:
     - master
 language: ruby
+cache: bundler
 rvm:
   - 2.3.7
   - 2.4.4
   - 2.5.1
-before_install: gem install bundler
+before_install:
+  - gem update --system
+  - gem install bundler
 install:
   - bundle install
 script: "bundle exec rake spec"