Update travis setting

Signed-off-by: Masahiro Nakagawa <[email protected]>

Author: repeatedly

.travis.yml

@@ -1,11 +1,10 @@
 language: ruby
 
 rvm:
-  - 2.1.10
-  - 2.2.9
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.4
+  - 2.5
+  - 2.6
+  - 2.7
   - ruby-head
 
 gemfile:
@@ -14,7 +13,6 @@ gemfile:
 branches:
   only:
     - master
-    - v0.12
 
 before_install: gem update bundler
 script: bundle exec rake test