Update ruby versions which use in CI

Kesin11 · Kesin11 · commit 0c6a611fda2f · 2020-07-05T00:11:11.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,8 +1,7 @@
 language: ruby
 rvm:
-  - 2.2.7
-  - 2.3.4
-  - 2.4.1
+  - 2.6.6
+  - 2.7.1
   - ruby-head
 script:
   - bundle exec rspec
