update(travis): Bump up go version to 1.12 to see if travis builds pass

.travis.yml

@@ -1,6 +1,6 @@
 sudo: false
 language: go
-go: 1.6
+go: 1.12
 script:
  - go get github.com/modocache/gover
  - go get github.com/prometheus/client_golang/...