Bump ruby in travis

Signed-off-by: Alex Pop <alexpop@users.noreply.github.com>

Author: alexpop

.travis.yml

@@ -1,8 +1,9 @@
----
+sudo: false
 language: ruby
 cache: bundler
+
 rvm:
-  - 2.3.3
+  - 2.4.1
 
 bundler_args: --without integration
 script: bundle exec rake