CI: Use same Ruby version matrix as Ruby

The Ruby support schedule currently contains 2.5-2.7. https://www.ruby-lang.org/en/downloads/branches/

Author: olleolleolle

.travis.yml

@@ -1,12 +1,10 @@
 language: ruby
-sudo: false
+
 rvm:
+- 2.7
+- 2.6
 - 2.5
-- 2.4
-- 2.3
-- 2.2
-- 2.1
-- jruby-9.1.7.0
+
 env:
   global:
   - PARSE_HOST=https://parse-ruby-client-server.herokuapp.com