Exclude builds for newer Rubies and old Rails versions

 * Rails 4.2 supports versions between 1.9 and 2.4: https://github.com/rails/rails/blob/406e2f46/.travis.yml#L26-L31
 * Rails 5.0 supports versions between 2.2 and 2.4: https://github.com/rails/rails/blob/ecd965e7/.travis.yml#L46-L48

Author: yuki24

.travis.yml

@@ -25,5 +25,13 @@ matrix:
       env: RAILS_VERSION=5.1
     - rvm: 2.2
       env: RAILS_VERSION=5.2
+    - rvm: 2.5
+      env: RAILS_VERSION=4.2
+    - rvm: 2.5
+      env: RAILS_VERSION=5.0
+    - rvm: 2.6
+      env: RAILS_VERSION=4.2
+    - rvm: 2.6
+      env: RAILS_VERSION=5.0
 
 sudo: false