Update README to install v7 with go get command

[jrh3k5]

The README's go get command won't install the latest version of the library - for example:

>go mod init graph-gophers-test
go: creating new go.mod: module graph-gophers-test
go: to add module requirements and sums:
        go mod tidy

>go get -u github.com/graph-gophers/dataloader
go: downloading github.com/graph-gophers/dataloader v5.0.0+incompatible
go: downloading github.com/opentracing/opentracing-go v1.2.0
go: added github.com/graph-gophers/dataloader v5.0.0+incompatible
go: added github.com/opentracing/opentracing-go v1.2.0

Contrast with the command in the README now:

>go mod init graph-gophers-test
go: creating new go.mod: module graph-gophers-test
go: to add module requirements and sums:
        go mod tidy

>go get -u github.com/graph-gophers/dataloader/v7
go: downloading github.com/graph-gophers/dataloader/v7 v7.1.0
go: added github.com/graph-gophers/dataloader/v7 v7.1.0