.travis.yml in cfndsl-1.3.1 vs .travis.yml in cfndsl-1.3.2

- old
+ new

@@ -1,7 +1,8 @@
 language: ruby
-rvm:
+rvm:  # all MRI ruby versions that are not EOL
 - 2.7
+- 2.6
 - '3.0'
 sudo: false
 before_install:
   - gem update --system
   - gem install bundler