travis: add go 1.5

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 9398694..5662541 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,4 +8,5 @@
   - 1.2
   - 1.3
   - 1.4
+  - 1.5
   - tip