Allow proc_macro_span feature

Workaround for dtolnay/proc-macro2#175

Author: Nemo157

.travis.yml

@@ -46,7 +46,7 @@ matrix:
     - name: cargo +stable build --no-default-features
       rust: nightly
       env:
-        - RUSTFLAGS="-Z allow-features="
+        - RUSTFLAGS="-Z allow-features=proc_macro_span" # TODO: https://github.com/rust-lang-nursery/futures-rs/issues/1564
       script:
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
         - cargo build --manifest-path futures/Cargo.toml --no-default-features
@@ -60,7 +60,7 @@ matrix:
     - name: cargo +stable build (alloc)
       rust: nightly
       env:
-        - RUSTFLAGS="-Z allow-features="
+        - RUSTFLAGS="-Z allow-features=proc_macro_span" # TODO: https://github.com/rust-lang-nursery/futures-rs/issues/1564
       script:
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
         - cargo build --manifest-path futures/Cargo.toml --no-default-features --features alloc
@@ -71,15 +71,15 @@ matrix:
     - name: cargo +stable build (default features)
       rust: nightly
       env:
-        - RUSTFLAGS="-Z allow-features="
+        - RUSTFLAGS="-Z allow-features=proc_macro_span" # TODO: https://github.com/rust-lang-nursery/futures-rs/issues/1564
       script:
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
         - cargo build --all
 
     - name: cargo +stable build (compat feature)
       rust: nightly
       env:
-        - RUSTFLAGS="-Z allow-features="
+        - RUSTFLAGS="-Z allow-features=proc_macro_span" # TODO: https://github.com/rust-lang-nursery/futures-rs/issues/1564
       script:
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
         - cargo build --manifest-path futures/Cargo.toml --features io-compat