Merge pull request #322 from intercom/sd/ruby-2.5.7

Use ruby 2.5.7 in CI

Author: sduquej

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: circleci/ruby:2.4.6
+      - image: circleci/ruby:2.5.7
 
     working_directory: ~/intercom-rails