Add Ruby 2.6, 2.7, 3.0 to Travis CI

Author: mattbrictson

.travis.yml

@@ -1,5 +1,8 @@
 language: ruby
 rvm:
+  - 3.0
+  - 2.7
+  - 2.6
   - 2.5
   - 2.4
   - 2.3
@@ -8,7 +11,7 @@ rvm:
   - 2.0
 branches:
   only:
-  - master
+    - master
 matrix:
   include:
     # Run Danger only once, on 2.5