update readme to specify latest version

Author: saghm

README.md

@@ -25,7 +25,7 @@ This repository contains the officially supported MongoDB Rust driver, a client
 The driver is available on [crates.io](https://crates.io/crates/mongodb). To use the driver in your application, simply add it to your project's `Cargo.toml`. You will also want to add [`bson`](https://docs.rs/bson) as well.
 ```toml
 [dependencies]
-mongodb = "0.9.0"
+mongodb = "0.9.1"
 bson = "0.14.0"
 ```