ruby_2_2 branch has ended

https://www.ruby-lang.org/en/news/2018/06/20/support-of-ruby-2-2-has-ended/
[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64783 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -83,7 +83,6 @@ script:
 branches:
   only:
     - trunk
-    - ruby_2_2
     - ruby_2_3
     - ruby_2_4
     - ruby_2_5