Bump Travis versions (#172)

diff --git a/.travis.yml b/.travis.yml
index 15e5a19..d4b9266 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@
   - 1.8.x
   - 1.9.x
   - 1.10.x
+  - 1.11.x
   - tip
 
 script: