CI against Ruby 2.5.0

Author: ttanimichi

.travis.yml

@@ -7,6 +7,7 @@ cache:
   bundler: true
   yarn: true
 rvm:
+  - 2.5.0
   - 2.3.0
   - 2.1
   - jruby-9.1.9.0
@@ -85,3 +86,4 @@ before_install:
   - echo "deb http://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
   - sudo apt-get update -qq
   - sudo apt-get install -y -qq yarn
+  - gem update --system