Update readme to 0.5.2

Author: mhallin

README.md

@@ -21,15 +21,15 @@ Add Juniper to your Cargo.toml:
 
 ```toml
 [dependencies]
-juniper = "0.5.1"
+juniper = "0.5.2"
 ```
 
 If you want the Iron integration enabled, you need to enable the `iron-handlers`
 feature flag:
 
 ```toml
 [dependencies]
-juniper = { version = "0.5.1", features = ["iron-handlers"] }
+juniper = { version = "0.5.2", features = ["iron-handlers"] }
 ```
 
 ## Building schemas