Release curl-sys 0.4.2 and curl 0.4.12 to publish working minimal dependency versions

[klausi]

Coming from rust-lang/cargo#5275

We need to get new versions of the crates out so that other crates can rely on the fixed minimum versions from #206

[matklad]

If we want to do a release due to version, then we might want to update winapi to 0.3 as well: #208

[alexcrichton]

Done!