`proc_macro_span` feature used on "stable" builds

[Nemo157]

With the changes from #1555 we are now attempting to build with something equivalent to what will be the 1.36 compiler. Unfortunately rust-lang/rust#60289 has revealed that proc-macro2 is using an unstable feature when it's built on nightly, which has broken those CI builds.

I've opened dtolnay/proc-macro2#175 to see if we can get a way to disable that feature in proc-macro2, but if we can't do that soon I'll probably revert the Travis changes from #1555 until it happens.

[taiki-e]

I think we can use RUSTFLAGS="-Z allow-features=proc_macro_span" temporarily.

[Nemo157]

@taiki-e great idea, I’ll open a PR in a bit.

[Nemo157]

#1565 added that workaround, leaving this open till we hear back about proc-macro2 changing so we can hopefully remove that.

[taiki-e]

(We probably can remove proc_macro_span from allow-features because dtolnay/proc-macro2#176 merged and proc-macro2 4.29 released.)