Update travis config.

Removing 1.5, since tests now use testing.Helper().
diff --git a/.travis.yml b/.travis.yml
index a60d535..f4cb25d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,6 @@
 sudo: false
 
 go:
-  - 1.5.x
   - 1.11.x
   - tip