GOPATH モードからモジュール対応モードへ移行せよ ref: https://qiita.com/spiegel-im-spiegel/items/5cb1587cb55d6f6a34d7

.goreleaser.yml

builds:
- env:
  - GO111MODULE=on

.travis.yml

language: go

go:
  - "1.11.x"

install:
  - env GO111MODULE=on go mod download

script:
  - env GO111MODULE=on go test ./...

after_success:
  - test -n "$TRAVIS_TAG" && curl -sL https://git.io/goreleaser | bash

file0.txt

$ go help mod
Go mod provides access to operations on modules.

Note that support for modules is built into all the go commands,
not just 'go mod'. For example, day-to-day adding, removing, upgrading,
and downgrading of dependencies should be done using 'go get'.
See 'go help modules' for an overview of module functionality.

Usage:

        go mod <command> [arguments]

The commands are:

        download    download modules to local cache
        edit        edit go.mod from tools or scripts
        graph       print module requirement graph
        init        initialize new module in current directory
        tidy        add missing and remove unused modules
        vendor      make vendored copy of dependencies
        verify      verify dependencies have expected content
        why         explain why packages or modules are needed

Use "go help mod <command>" for more information about a command.

file10.txt

$ go list -m -u all
go: finding github.com/pkg/errors v0.8.0
go: finding github.com/spf13/pflag v1.0.2
go: finding github.com/spf13/cobra v0.0.3
go: finding github.com/BurntSushi/toml v0.3.0
go: finding github.com/spiegel-im-spiegel/gocli v0.8.0
go: finding github.com/inconshreveable/mousetrap v1.0.0
go: finding golang.org/x/crypto v0.0.0-20180816225734-aabede6cba87
go: finding golang.org/x/crypto latest
go: finding github.com/BurntSushi/toml v0.3.1
github.com/spiegel-im-spiegel/gpgpdump
github.com/BurntSushi/toml v0.3.0 [v0.3.1]
github.com/inconshreveable/mousetrap v1.0.0
github.com/pkg/errors v0.8.0
github.com/spf13/cobra v0.0.3
github.com/spf13/pflag v1.0.2
github.com/spiegel-im-spiegel/gocli v0.8.0
golang.org/x/crypto v0.0.0-20180816225734-aabede6cba87 [v0.0.0-20180910181607-0e37d006457b]

file17.txt

$ go run md2html.go hello.md
go: finding gopkg.in/russross/blackfriday.v2 v2.0.1
go: gopkg.in/russross/blackfriday.v2@v2.0.1: go.mod has non-....v2 module path "github.com/russross/blackfriday/v2" at revision v2.0.1
go: error loading module requirements

file2.txt

$ go mod init hello
go: creating new go.mod: module hello

file20.txt

$ go run md2html.go hello.md
go: finding github.com/russross/blackfriday/v2 v2.0.1
go: downloading github.com/russross/blackfriday/v2 v2.0.1
go: finding github.com/shurcooL/sanitized_anchor_name latest
go: downloading github.com/shurcooL/sanitized_anchor_name v0.0.0-20170918181015-86672fcb3f95
<h1>Hello</h1>

<p>Hello, World!</p>

file4.txt

$ go run hello.go
go: finding rsc.io/quote v1.5.2
go: downloading rsc.io/quote v1.5.2
go: finding rsc.io/sampler v1.3.0
go: finding golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
go: downloading rsc.io/sampler v1.3.0
go: downloading golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
Hello, world.

file6.txt

$ go mod graph
hello rsc.io/quote@v1.5.2
rsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0
rsc.io/sampler@v1.3.0 golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c

file9.txt

$ go mod init github.com/spiegel-im-spiegel/gpgpdump
go: creating new go.mod: module github.com/spiegel-im-spiegel/gpgpdump
go: copying requirements from Gopkg.lock

go.mod

module md2html

require (
    github.com/shurcooL/sanitized_anchor_name v0.0.0-20170918181015-86672fcb3f95 // indirect
    gopkg.in/russross/blackfriday.v2 v2.0.1
)

replace gopkg.in/russross/blackfriday.v2 v2.0.1 => github.com/russross/blackfriday/v2 v2.0.1

hello.go

package main

import (
    "fmt"

    "rsc.io/quote"
)

func main() {
    fmt.Println(quote.Hello())
}

md2html.go

package main

import (
    "fmt"
    "io/ioutil"
    "os"

    "gopkg.in/russross/blackfriday.v2"
)

func main() {
    md, err := ioutil.ReadFile(os.Args[1])
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        return
    }

    html := blackfriday.Run(md)
    fmt.Println(string(html))
}