Auto merge of #2519 - alexcrichton:update-curl, r=alexcrichton
Auto merge of #2519 - alexcrichton:update-curl, r=alexcrichton

Update curl-sys

Picks up a fix to hopefully and correctly configure OpenSSL to be enabled in
cross-compiled situations where OpenSSL comes from a different location
(currently specified by the `OPENSSL_ROOT_DIR` environment variable that libssh2
also reads).

achanda/cargo,rustyhorde/cargo,durka/cargo,tshepang/cargo,AndiDog/cargo,blaenk/cargo,mark-buer/cargo,tsurai/cargo,brson/cargo,nipunn1313/cargo,tsurai/cargo,AndiDog/cargo,vojtechkral/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,brson/cargo,durka/cargo,AndiDog/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,tshepang/cargo,tshepang/cargo,brson/cargo,eminence/cargo,mark-buer/cargo,rust-lang/cargo,mark-buer/cargo,rustyhorde/cargo,AndiDog/cargo,rustyhorde/cargo,achanda/cargo,blaenk/cargo,mark-buer/cargo,blaenk/cargo,nipunn1313/cargo,durka/cargo,sbeckeriv/cargo,blaenk/cargo,eminence/cargo,mark-buer/cargo,sbeckeriv/cargo,nipunn1313/cargo,brson/cargo,achanda/cargo,durka/cargo,sbeckeriv/cargo,vojtechkral/cargo,vojtechkral/cargo,brson/cargo,tshepang/cargo,tsurai/cargo,sbeckeriv/cargo,AndiDog/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,achanda/cargo,nipunn1313/cargo,blaenk/cargo