ci: Update CI config to build async-await feature on beta

Author: taiki-e

.travis.yml

@@ -20,6 +20,14 @@ matrix:
         - cargo build --all
         - cargo build --manifest-path futures/Cargo.toml --features io-compat
 
+    # This is the minimum Rust version supported by `async-await` feature.
+    # When updating this, the reminder to update the minimum required version of `async-await` feature in README.md.
+    - name: cargo build --features async-await (minimum required version)
+      rust: beta # 1.39.0
+      script:
+        - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
+        - cargo build --all --features async-await
+
     - name: cargo +stable build
       rust: stable
       script:
@@ -31,18 +39,11 @@ matrix:
     - name: cargo +beta build
       rust: beta
       script:
-        # default features & compat feature
+        # default features & compat feature & async-await feature
         - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
         - cargo build --all
         - cargo build --manifest-path futures/Cargo.toml --features io-compat
-
-    # This is the minimum Rust version supported by `async-await` feature.
-    # When updating this, the reminder to update the minimum required version of `async-await` feature in README.md.
-    - name: cargo +nightly build (minimum required version)
-      rust: nightly-2019-08-21
-      script:
-        - cargo run --manifest-path ci/remove-dev-dependencies/Cargo.toml */Cargo.toml
-        - cargo build --all --all-features
+        - cargo build --manifest-path futures/Cargo.toml --features async-await
 
     - name: cargo test
       rust: nightly

README.md

@@ -65,7 +65,7 @@ The `async-await` feature provides several convenient features using async/await
 futures-preview = { version = "=0.3.0-alpha.19", features = ["async-await"] }
 ```
 
-The current `async-await` feature requires Rust nightly 2019-08-21 or later.
+The current `async-await` feature requires Rust 1.39 or later.
 
 # License