Update average requirement from 0.14 to 0.15 (#1426)

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rand_distr/Cargo.toml

@@ -37,6 +37,6 @@ rand_pcg = { version = "=0.9.0-alpha.1", path = "../rand_pcg" }
 # For inline examples
 rand = { path = "..", version = "=0.9.0-alpha.1", features = ["small_rng"] }
 # Histogram implementation for testing uniformity
-average = { version = "0.14", features = [ "std" ] }
+average = { version = "0.15", features = [ "std" ] }
 # Special functions for testing distributions
 special = "0.10.3"