Adjust readme to not bump to 1.6

Author: sfackler

README.md

@@ -54,7 +54,7 @@ fn main() {
 ```
 
 ## Requirements
-* **Rust** - Rust-Postgres is developed against the 1.5 release of Rust
+* **Rust** - Rust-Postgres is developed against the 1.6 release of Rust
     available on http://www.rust-lang.org. It should also compile against more
     recent releases.