.travis.yml in drg-1.5.2 vs .travis.yml in drg-1.6.0

@@ -1,10 +1,7 @@
 language: ruby
 rvm:
-  - 2.0.0
-  - 2.1.0
-  - 2.1.5
-  - 2.2.0
-  - 2.2.1
-  - 2.2.2
-  - 2.2.3
-before_install: gem install bundler -v 1.10.5
+  - 2.3.8
+  - 2.4.8
+  - 2.5.6
+  - 2.6.5
+before_install: gem install bundler