Support Latest Ruby versions

2.3 and 2.4 are EOL now, and 2.7 officially exists

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

Author: Daniel Magliola

.travis.yml

@@ -5,8 +5,7 @@ before_install:
   - |
     if [[ "$(ruby -e 'puts RUBY_VERSION')" != 1.* ]]; then gem update --system; fi
 rvm:
-  - 2.3.8
-  - 2.4.5
-  - 2.5.3
-  - 2.6.0
+  - 2.5.8
+  - 2.6.6
+  - 2.7.1
   - jruby-9.1.9.0