Building SSH CA fails with "go-md2man" package not found

I was trying to build the current master branch as per https://github.com/cloudtools/ssh-cert-authority/blob/master/BUILDING.rst.

After executing ...
```
docker run \
-v `pwd`:/build/ssh-cert-authority/go/src/github.com/cloudtools/ssh-cert-authority \
-t cloudtools/ssh-cert-authority-buildenv \
bash build.sh
```

... the build process fails with the following error message after a few minutes:

```
package github.com/cpuguy83/go-md2man/v2/md2man: cannot find package "github.com/cpuguy83/go-md2man/v2/md2man" in any of:
/build/go-build/go/src/github.com/cpuguy83/go-md2man/v2/md2man (from $GOROOT)
/build/ssh-cert-authority/go/src/github.com/cpuguy83/go-md2man/v2/md2man (from $GOPATH)
../../codegangsta/cli/docs.go:11:2: cannot find package "github.com/cpuguy83/go-md2man/v2/md2man" in any of:
/build/go-build/go/src/github.com/cpuguy83/go-md2man/v2/md2man (from $GOROOT)
/build/ssh-cert-authority/go/src/github.com/cpuguy83/go-md2man/v2/md2man (from $GOPATH)
Makefile:54: recipe for target 'test' failed
make: *** [test] Error 1
```

It seems like v2 of go-md2man was finally released and is not available as a separate branch anymore.