Update arc-swap dependency to 0.4.8

This patch updates the arc-swap dependency from 0.4.7 to 0.4.8 as the
old version has been yanked.

Cargo.lock

@@ -34,9 +34,9 @@ checksum = "85bb70cc08ec97ca5450e6eba421deeea5f172c0fc61f78b5357b2a8e8be195f"
 
 [[package]]
 name = "arc-swap"
-version = "0.4.7"
+version = "0.4.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4d25d88fd6b8041580a654f9d0c581a047baee2b3efee13275f2fc392fc75034"
+checksum = "dabe5a181f83789739c194cbe5a897dde195078fac08568d09221fd6137a7ba8"
 
 [[package]]
 name = "array-macro"