CI against ruby 2.6

Author: amatsuda

.travis.yml

@@ -16,6 +16,7 @@ rvm:
   - 2.3.8
   - 2.4.6
   - 2.5.5
+  - 2.6.3
 
 gemfile:
   - Gemfile
@@ -75,6 +76,16 @@ matrix:
       gemfile: gemfiles/Gemfile-4-0-stable
     - rvm: 2.5.5
       gemfile: gemfiles/Gemfile-4-1-stable
+    - rvm: 2.6.3
+      gemfile: gemfiles/Gemfile-4-0-stable
+    - rvm: 2.6.3
+      gemfile: gemfiles/Gemfile-4-1-stable
+    - rvm: 2.6.3
+      gemfile: gemfiles/Gemfile-4-2-stable
+    - rvm: 2.6.3
+      gemfile: gemfiles/Gemfile-5-0-stable
+    - rvm: 2.6.3
+      gemfile: gemfiles/Gemfile-5-1-stable
 
 notifications:
   email: false