[travis] Bump go version to 1.6
diff --git a/.travis.yml b/.travis.yml
index 295f0c3..c47256e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,8 @@
 language: go
 
+go:
+  - 1.6
+
 install:
   # go-flags
   - go get -d -v ./...