Update system gems on travis.

Author: gkellogg

.travis.yml

@@ -1,13 +1,14 @@
 language: ruby
 bundler_args: --without debug
 script: "bundle exec rspec spec"
+before_install: "gem update --system"
 env:
   - CI=true
 rvm:
   - 2.2
   - 2.3
   - 2.4
-  - 2.5.0
+  - 2.5
   - jruby
   - rbx-3
 cache: bundler