Test gem against Ruby 2.7.

Test gem against Ruby 2.7.

diff --git a/.travis.yml b/.travis.yml
index c2aa444..e656bf1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,6 +10,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
+  - 2.7
 
 matrix:
   fast_finish: true

GitHub sha: 37203967