Update documented requirements (#483)

Tried to install the current version of this cookbook with Chef 12, and got this error:

```
Cookbook 'aws' version '9.1.4' depends on chef version [">= 15.3"], but the running chef version is 12.18.31
```

README.md

@@ -69,7 +69,7 @@ This cookbook is maintained by the Sous Chefs. The Sous Chefs are a community of
 
 ### Chef
 
-- Chef 12.9+
+- Chef 15.3+
 
 ### Cookbooks