Test against Ruby 2.6 (#382)

Test against Ruby 2.6 (#382)

diff --git a/.travis.yml b/.travis.yml
index 210c1a4..08b4bb3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,7 @@ rvm:
   - 2.3
   - 2.4
   - 2.5
+  - 2.6
 before_install:
   - gem install bundler
 services:

GitHub sha: 9b604bd3