tech(build): switch to golang 1.7.3

tls/cipher fixes
https://github.com/golang/go/issues?q=milestone%3AGo1.7.3

circle.yml

@@ -5,7 +5,7 @@ machine:
     GOPATH: "/home/ubuntu/.go_workspace"
     ORG_PATH: "github.com/grafana"
     REPO_PATH: "${ORG_PATH}/grafana"
-    GODIST: "go1.7.1.linux-amd64.tar.gz"
+    GODIST: "go1.7.3.linux-amd64.tar.gz"
   post:
     - mkdir -p download
     - test -e download/$GODIST || curl -o download/$GODIST https://storage.googleapis.com/golang/$GODIST