Fix command for installing godep in readme
Fix command for installing godep in readme

The readme file says to run `go install github.com/tools/godep` to install godep. However, running this in my development VM gave the following error message:

```
can't load package: package github.com/tools/godep: cannot find package "github.com/tools/godep" in any of:
	/usr/lib/goenv/versions/1.6.3/src/github.com/tools/godep (from $GOROOT)
	/var/govuk/gopath/src/github.com/tools/godep (from $GOPATH)
```

Looking at https://github.com/tools/godep/blob/master/Readme.md, the command it suggests is `go get github.com/tools/godep`, which works.

Is this simply a change in naming in newer versions of the `go` command, or am I missing something here?
alphagov/router