*: bump default Go version to 1.6.2

.promu.yml

-go: 1.5.4
+go: 1.6.2
 repository:
     path: github.com/prometheus/prometheus
 build:

.travis.yml

@@ -3,7 +3,8 @@ sudo: false
 language: go
 
 go:
-- 1.5
+- 1.5.4
+- 1.6.2
 
 script:
 - make check_license style test