Use BN_div instead of BN_mod

BN_mod is not available on all plateform and can be replaced by BN_div with dv set as NULL.

I had an error from the linker about `_BN_mod` not found when I tried building rust-openssl. It seems like openssl 1.0.1g on OS X dosn't contain BN_mod. Will check if it is a bug in OpenSSL OS X build.

Meanwhile, an easy fix is to use instead BN_div as explained in the [man page](https://www.openssl.org/docs/crypto/BN_add.html):

    BN_mod() corresponds to BN_div() with dv set to NULL.