README.md: require sbt >= 1.5.3

Author: smarter

README.md

@@ -30,10 +30,10 @@ You will need to make the following adjustments to your build:
 ### project/build.properties
 
 ```scala
-sbt.version=1.5.0
+sbt.version=1.5.3
 ```
 
-You must use sbt 1.5.0 or newer; older versions of sbt are not supported.
+You must use sbt 1.5.3 or newer; older versions of sbt are not supported.
 
 ### build.sbt