Update tower requirement from 0.4 to 0.5 #45

Job	Run time
Audit	6s
Minimal Versions	2m 46s
Clippy	20s
Rustdoc	51s
Minimal Versions (`tls-rustls`)	2m 57s
Build	16s
Clippy (`tls-rustls`)	36s
Minimal Versions (`tls-rustls-no-provider`)	2m 51s
Rustdoc with `cfg(docsrs)`	57s
Clippy (`tls-rustls-no-provider`)	20s
Build (`tls-rustls`)	34s
Minimal Versions (`tls-openssl`)	3m 18s
Clippy (`tls-openssl`)	28s
Build (`tls-rustls-no-provider`)	17s
Minimal Versions (`--all-features`)	3m 17s
Rustfmt	3s
Clippy (`--all-features`)	44s
Build (`tls-openssl`)	20s
Build (`--all-features`)	46s
Test	31s
Build MSRV	2m 32s
Test (`tls-rustls`)	48s
Build MSRV (`tls-rustls`)	3m 2s
Build MSRV (`tls-rustls-no-provider`)	2m 39s
Build MSRV (`tls-openssl`)	2m 41s
Test (`rustls/ring`)	30s
Build MSRV (`--all-features`)	2m 58s
Test (`tls-openssl`)	34s
Test (`--all-features`)	54s
	38m 56s

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