Explain what "latest libgit2" means better

pygit2 targets the master branch, and "latest" isn't well-defined in
this case, as libgit2's default branch is development.

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This looks to be the cause of confusion in http://stackoverflow.com/questions/16570217/installing-pygit2-libgit2-in-python-virtualenv-on-a-mac and being explicit about what we target about is good anyway.