Use go 1.13

Use go 1.13 for testing in CI. This makes it possible to use
go 1.13 functionality. This does also mean, that at least go 1.13
is needed to build from source.

Author: SimonBarendse

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   test:
     docker:
-      - image: circleci/golang:1.12
+      - image: circleci/golang:1.13
     steps:
       - checkout
       - restore_cache:

go.mod

@@ -1,6 +1,6 @@
 module github.com/secrethub/secrethub-cli
 
-go 1.12
+go 1.13
 
 require (
 	bitbucket.org/zombiezen/cardcpx v0.0.0-20150417151802-902f68ff43ef