go mod: set version to 1.16 & drop explicit "-mod=vendor" from ci

This makes "go build" automatically use the vendor
directory, if present.

See https://golang.org/doc/modules/gomod-ref#go for details.

Up to now, we ignored the vendor dir completely!

Fixes https://github.com/rfjakob/gocryptfs/issues/581

Makefile

@@ -42,6 +42,6 @@ ci:
 	go clean -modcache
 	# GOPROXY=off makes sure we fail instead of making network requests
 	# (we should not need any!)
-	GOPROXY=off ./build.bash -mod=vendor
+	GOPROXY=off ./build.bash
 	# Delete "vendor" dir
 	git clean -dxff

go.mod

@@ -1,6 +1,6 @@
 module github.com/rfjakob/gocryptfs
 
-go 1.13
+go 1.16
 
 require (
 	github.com/hanwen/go-fuse/v2 v2.1.1-0.20210611132105-24a1dfe6b4f8

go.sum

@@ -42,7 +42,6 @@ golang.org/x/sys v0.0.0-20200323222414-85ca7c5b95cd/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20200501145240-bc7a7d42d5c3 h1:5B6i6EAiSYyejWfvc5Rc9BbI3rzIsrrXfAQBWnYfn+w=
 golang.org/x/sys v0.0.0-20200501145240-bc7a7d42d5c3/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=