.travis.yml in kindle_manager-0.6.2 vs .travis.yml in kindle_manager-0.6.3

@@ -1,7 +1,7 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.2.7
-  - 2.3.4
-  - 2.4.1
-before_install: gem install bundler -v 1.14.6
+  - 2.4.5
+  - 2.5.3
+  - 2.6.0
+before_install: gem install bundler