travis: add go 1.6

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 5662541..5c7621e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,4 +9,5 @@
   - 1.3
   - 1.4
   - 1.5
+  - 1.6
   - tip