Run tests against Ruby 2.6 on CI.

Run tests against Ruby 2.6 on CI.

diff --git a/.travis.yml b/.travis.yml
index d23fef9..d20748c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,7 @@ rvm:
   - 2.3
   - 2.4
   - 2.5
+  - 2.6
 gemfile:
   - Gemfile
 addons:

GitHub sha: 00826062