Fix breakage caused by builder trait removal #994

Job	Run time
Build docs	1m 21s
Clippy	47s
Test (MSRV)	1m 22s
Rustfmt	5s
Test (stable)	2m 1s
Test (beta)	2m 0s
Test (nightly)	1m 56s
Test (macOS)	4m 9s
Test (Windows)	4m 15s
Test (no features)	1m 8s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 14s
	20m 18s

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