Update version for 0.4.0 release

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "option-operations"
-version = "0.3.99"
+version = "0.4.0"
 authors = ["François Laignel <[email protected]>"]
 categories = ["rust-patterns", "no-std", "mathematics"]
 description = "Traits and auto-implementations to improve arithmetic operations usability when dealing with `Option`s."
@@ -10,8 +10,6 @@ readme = "README.md"
 repository = "https://github.com/fengalin/opt-operations"
 edition = "2018"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [features]
 default = ["std"]