Removed "nightly" from default features

[rozbb]

I was using this crate and was surprised to have a compile error when switching to "stable". The error came from subtle trying to use a #![feature(...)] gate. I spent some time tracking down what was using subtle with the "nightly" feature flag set. I checked curve25519-dalek and ed25519-dalek and neither of them set "nightly" by default. I found it a bit surprising, then, that x25519-dalek was the culprit.

So, in keeping with curve25519-dalek, ed25519-dalek, subtle, and zkp, I think this should compile by default on "stable".

[hdevalence]

This looks good, thanks! The CI failure seems unrelated to your changes and instead related to the usual rand brokenness (rust-random/rand#704)

[hdevalence]

Released as 0.4.5.