Making the submodule accessible behind a proxy (via https://)

[peter-kehl]

Hi Vincent,

Thank you for GitFlow.

Currently, people can't recursively clone GitFlow behind a proxy. Even if they run

git clone --recursive https://github.com/nvie/gitflow.git

the submodule was accessed via git:// - (behind a proxy) it fails.

In addition to this pull request, would you update https://github.com/nvie/gitflow/wiki/Manual-installation:

change from
git clone --recursive git://github.com/nvie/gitflow.git

to
git clone --recursive https://github.com/nvie/gitflow.git

This may be related to #104 - I'm not sure.