.travis.yml in mb_string-0.1.7 vs .travis.yml in mb_string-0.1.8

@@ -1,6 +1,7 @@
 language: ruby
 rvm:
-  - 2.0.0
-  - 2.1.7
-  - 2.2.3
-before_install: gem install bundler -v 1.10.6
+  - 2.1.10
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
+# before_install: gem install bundler -v 1.10.6