Remove reference to the nightly feature from crate docs

[KodrAus]

Following up for #136

Just removes a reference to the nightly feature (that was removed in #138) from the crate docs.

I thought I'd leave the CI jobs for building and checking on nightly just because it's nice to know whether the nightly channel manages to break in some way so that can get reported back upstream.

cc @Ericson2314

[Ericson2314]

Ah so the thing about the nightly jobs is that they do more. The travis one has it alone generate docs, the appveyor one has it alone do some sort of a tests. I agree testing should happen on nightly, but I don't think it should be so distinguished.

[KodrAus]

I think we need to use the nightly channel for the compile tests, because the infrastructure that runs them requires it.

As for docs generation, I wonder if we need to do it all now that docs.rs is ubiquitous. I guess it doesn't hurt.

[BurntSushi]

As for docs generation, I wonder if we need to do it all now that docs.rs is ubiquitous. I guess it doesn't hurt.

FWIW, I stopped doing it for the regex crate and just rely on docs.rs. So if you need precedent, feel free to use me. :-)