Update circleci/golang Docker tag to v1.12.6 (#68)

renovate[bot] · Yusuke Kato · commit 3a738d7f4d8e · 2019-06-13T09:47:05.000+09:00
diff --git a/.circleci/config.yml b/.circleci/config.yml
@@ -2,7 +2,7 @@ alias:
   default: &default
     working_directory: /go/src/github.com/kpango/gache
     docker:
-      - image: circleci/golang:1.12.5
+      - image: circleci/golang:1.12.6
         environment:
           GOPATH: "/go"
           GO111MODULE: "on"
