Auto merge of #598 - saethlin:vendored-libgit, r=Mark-Simulacrum

Bump libgit2-sys version to be compatible with systems that have newer libgit2

`cargo run -- prepare-local` doesn't work at the moment on Arch/Manjaro, because rolling release distros are shipping `libgit` 1.3.0, which has a compatability break (intentional or not it's unclear) since 1.1.0. `libgit2-sys` will build a vendored version if the system version is too old, but not if it is too new. So with this change, the build should be correct on all systems.

~~I'm running into the same error as rustsec/rustsec#431 and applying the fix suggested therein.~~

Author: bors