Update .gitmodules 'greg' URL to https://

Updated 'greg' to use https:// URL instead of git://. The git:// URL can break support for users behind firewall or proxy and use of https:// is consistent with the other submodules, "MarkdownTest" and "Support". This will also not allow some users to install the homebrew MultiMarkdown-4 and this URL can only be edited in the MultiMarkdown-4 repo, not the homebrew formula.