CI: Use ruby 2.4.9, 2.5.7, 2.6.5 and rails 6.0.0

Author: mikong

.travis.yml

@@ -1,20 +1,20 @@
 language: ruby
 cache: bundler
 rvm:
-  - 2.4.6
-  - 2.5.5
-  - 2.6.3
+  - 2.4.9
+  - 2.5.7
+  - 2.6.5
   - ruby-head
 env:
   - RAILS_VERSION="~> 4.2.0"
   - RAILS_VERSION="~> 5.0.0"
   - RAILS_VERSION="~> 5.1.0"
   - RAILS_VERSION="~> 5.2.0"
-  - RAILS_VERSION="~> 6.0.0.rc1"
+  - RAILS_VERSION="~> 6.0.0"
 matrix:
   exclude:
     - rvm: 2.4.6
-      env: RAILS_VERSION="~> 6.0.0.rc1"
+      env: RAILS_VERSION="~> 6.0.0"
     - rvm: 2.6.3
       env: RAILS_VERSION="~> 4.2.0"
   allow_failures: