p2p: Update arbitrary dep to 1.4.1

I don't know why `cargo` can't work this out but `just
update-lock-files` is throwing an error

```
error: failed to select a version for `arbitrary`.
    ... required by package `bitcoin-p2p-messages v0.1.0 (/home/tobin/build/rust-bitcoin/p2p)`
    ... which satisfies path dependency `p2p` of package `bitcoin-fuzz v0.0.1 (/home/tobin/build/rust-bitcoin/fuzz)`
versions that meet the requirements `^1.4` are: 1.4.0

all possible versions conflict with previously selected packages.

  previously selected package `arbitrary v1.4.1`
    ... which satisfies dependency `arbitrary = "^1.4.1"` of package `bitcoin-fuzz v0.0.1 (/home/tobin/build/rust-bitcoin/fuzz)`
```

Just update the arbitrary dep to `v1.4.1` like in the other crates.

Author: tcharding

p2p/Cargo.toml

@@ -25,7 +25,7 @@ internals = { package = "bitcoin-internals", path = "../internals", default-feat
 io = { package = "bitcoin-io", path = "../io", default-features = false }
 units = { package = "bitcoin-units", path = "../units", default-features = false }
 
-arbitrary = { version = "1.4", optional = true }
+arbitrary = { version = "1.4.1", optional = true }
 
 [dev-dependencies]
 hex_lit = "0.1.1"