.travis.yml in slack-ruby-client-0.14.6 vs .travis.yml in slack-ruby-client-0.15.0

@@ -1,10 +1,9 @@
 language: ruby
 
 cache: bundler
 
 rvm:
-  - 2.3.8
   - 2.5.3
   - 2.6.0
   - ruby-head
   - jruby-head