update circle.yml to use golang1.8

circle.yml

@@ -1,6 +1,13 @@
 machine:
   timezone:
     Asia/Tokyo
+  environment:
+    GODIST: "go1.8.1.linux-amd64.tar.gz"
+  post:
+    - mkdir -p downloads
+    - test -e download/$GODIST || curl -o download/$GODIST https://storage.googleapis.com/golang/$GODIST
+    - sudo rm -rf /usr/local/go
+    - sudo tar -C /usr/local -xzf download/$GODIST
 test:
   override:
     - GOOS=linux GOARCH=amd64 go test -v ./...