Raise minimum rustc support to 1.31

Author: chenyh1990

README.md

@@ -105,7 +105,7 @@ so it may not be a huge problem for most users.
 
 ## Minimum Rust version policy
 
-This crate's minimum supported `rustc` version is 1.28, as this is the currently known minimum version for [`regex`](https://crates.io/crates/regex#minimum-rust-version-policy) as well.
+This crate's minimum supported `rustc` version is 1.31 (released with Rust 2018, after v0.4.3), as this is the currently known minimum version for [`regex`](https://crates.io/crates/regex#minimum-rust-version-policy) as well.
 
 ## License