fix(cargo): Update minimal dependency versions so that 'cargo build -Z minimal-versions' works

[klausi]

The crate does not compile with cargo build -Z minimal-versions because a lot of minimal versions are underspecified. I bumped them until I could build successfully.

Ideally we should also add something like cargo update -Z minimal-versions && cargo build to the Travis CI config so that this does not happen again in the future. I'm not sure where to put that exactly?

[alexcrichton]

Thanks so much!

Sounds like a great idea to me to add this to CI eventually (and I think long-term we'd even do this on cargo publish). For the time being though I think it's ok to skip that as it's still a nightly Cargo feature and otherwise this lib isn't changing a whole lot