Update .travis.yml

 * Do not test against ruby-head and old Rails versions
 * Do not test against old Rubies and newer Rails versions
 * Use latest minor Rubies

.travis.yml

@@ -11,11 +11,11 @@ before_install:
 
 rvm:
   - 2.3.8
-  - 2.4.5
-  - 2.5.3
-  - 2.6.1
+  - 2.4.6
+  - 2.5.5
+  - 2.6.3
   - ruby-head
-  - jruby-9.2.5.0
+  - jruby-9.2.7.0
   - jruby-head
 
 gemfile:
@@ -28,12 +28,24 @@ gemfile:
 
 matrix:
   exclude:
+    - rvm: 2.3.8
+      gemfile: gemfiles/rails_60.gemfile
     - rvm: 2.3.8
       gemfile: gemfiles/rails_edge.gemfile
-    - rvm: 2.4.5
+    - rvm: 2.4.6
+      gemfile: gemfiles/rails_60.gemfile
+    - rvm: 2.4.6
       gemfile: gemfiles/rails_edge.gemfile
+    - rvm: 2.6.3
+      gemfile: gemfiles/rails_42.gemfile
+    - rvm: ruby-head
+      gemfile: gemfiles/rails_42.gemfile
+    - rvm: ruby-head
+      gemfile: gemfiles/rails_50.gemfile
+    - rvm: ruby-head
+      gemfile: gemfiles/rails_51.gemfile
   allow_failures:
     - rvm: ruby-head
-    - rvm: jruby-9.2.5.0
+    - rvm: jruby-9.2.7.0
     - rvm: jruby-head
     - gemfile: gemfiles/rails_edge.gemfile