Building with stable rust

[plietar]

Since librespot was started rust has stabilized a lot of stuff. As a result where lots of unstable features were required before, only zero_one, iter_arith, and plugin are needed now.

The first two are trivial to work around.
The last one not so much. The two plugins used are json_macros, which could easily be avoided, and protobuf_macros which is used a lot and would be a pain to get rid, both in terms of work and readability.

syntex would be a potential solution to this.

[plietar]

Building on rust stable now works (d27c4ca).
I'm keeping this open until I update the README.

[herrernst]

This is great news! There is one last gotcha when building with discovery: plietar/rust-dns-sd#2
Thank you!

[plietar]

Oops, good catch thanks.
I probably need to update Cargo.lock as well, I'll do that when I get back this evening.

[plietar]

README has been updated, with instructions on how to build without syntex.
Travis also builds using stable, beta and nightly, making sure stable support doesn't break unexpectedly.