.travis.yml in rakuten_web_service-1.7.0 vs .travis.yml in rakuten_web_service-1.8.0

@@ -1,11 +1,13 @@
 language: ruby
 rvm:
-  - 2.1.10
-  - 2.2.6
-  - 2.3.3
-  - 2.4.0
+  - 2.2.9
+  - 2.3.6
+  - 2.4.3
+  - 2.5.0
   - ruby-head
+
+group: travis_lts
 
 cache: bundler
 
 before_install:
   - gem install bundler