obfs4 needs golang.org/x/net/http2

The nightly build from today fails to build obfs4 with the following error:

/var/tmp/dist/gopath/src/gitlab.com/yawning/obfs4.git/transports/meeklite/transport.go:31:2: cannot find package "golang.org/x/net/http2" in any of:
        /var/tmp/dist/go/src/golang.org/x/net/http2 (from $GOROOT)
        /var/tmp/dist/gopath/src/golang.org/x/net/http2 (from $GOPATH)

I am not sure why the previous build did not fail. This might be related to the changes from legacy/trac#29178 (moved).