go mod tidy (#1464)

* go mod tidy

was getting checksum mismatches, this fixes

* update circleci to build w/ go1.12.1

Author: rdallman

.circleci/config.yml

@@ -6,7 +6,7 @@ jobs:
     working_directory: ~/go/src/github.com/fnproject/fn
     environment: # apparently expansion doesn't work here yet: https://discuss.circleci.com/t/environment-variable-expansion-in-working-directory/11322
       - GOPATH=/home/circleci/go
-      - GOVERSION=1.11.5
+      - GOVERSION=1.12.1
       - OS=linux
       - ARCH=amd64
       - FN_LOG_LEVEL=debug