Add Go 1.9 to Travis testers (#365)

diff --git a/.travis.yml b/.travis.yml
index 09fb310..47af085 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
 go:
   - 1.7.x
   - 1.8.x
+  - 1.9.x
   - master
 
 install: