execute command goreportcard-cli -v will output errors

[windvalley]

Describe the bug
When I use goreportcard-cli -v command to check the code quality of my Go project, the following error will be output:

Grade: A+ (99.6%)
Files: 73
Issues: 1
gofmt: 100%
go_vet: 98%
	router/init.go
		Line 21: error: package project-foo/docs is not in GOROOT (/usr/local/Cellar/go/1.15/libexec/src/project-foo/docs) (vet)
gocyclo: 100%
golint: 100%
ineffassign: 100%
license: 100%
misspell: 100%

To Reproduce
Steps to reproduce the behavior:

1. install goreportcard-cli

brew tap alecthomas/homebrew-tap
brew install gometalinter
go get github.com/gojp/goreportcard/cmd/goreportcard-cli

2. swag init to generate dir docs/
3. goreportcard-cli -v at Go project root dir
4. See the errors above

Expected behavior

Grade: A+ (100.0%)
Files: 74
Issues: 0
gofmt: 100%
go_vet: 100%
gocyclo: 100%
golint: 100%
ineffassign: 100%
license: 100%
misspell: 100%

Your swag version
v1.7.0

Your go version
go version go1.15 darwin/amd64

[ubogdan]

Merged.