Link to openssl statically for nightlies

An explanation can be found in the commit I made to git2-rs [1]. This is the
first step towards dealing with #457.

[1]: https://github.com/alexcrichton/git2-rs/commit/16142ef9
auto merge of #459 : alexcrichton/cargo/update-git, r=alexcrichton

An explanation can be found in the commit I made to git2-rs [1]. This is the
first step towards dealing with #457.

[1]: https://github.com/alexcrichton/git2-rs/commit/16142ef9