Upgrade rand to 0.7

Author: upsuper

phf_generator/Cargo.toml

@@ -8,5 +8,5 @@ repository = "https://github.com/sfackler/rust-phf"
 edition = "2018"
 
 [dependencies]
-rand = "0.6"
+rand = { version = "0.7", features = ["small_rng"] }
 phf_shared = { version = "0.7.24", path = "../phf_shared" }