.travis.yml: update stable go version to 1.5
diff --git a/.travis.yml b/.travis.yml
index 4e76534..90c9c6f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,5 @@
 language: go
 go:
-  - 1.4
+  - 1.5
   - tip