Update for breaking changes in underlying rustc.

This should be a major version number since I changed public constants
in the ffi module. I'm not doing so as the invariant "will the constants
be meaningful to the underlying library" has not changed.

In general this library's version numbers do not map well to the
underlying library, which is as-yet not versioned at all, so users
need to always be running "the lastest" rust-secp256k1 anyway, and
semantic versioning can't really be used meaninfully. So this is a
bit of a judgement call.