Update Bitcoin Core version expected

Author: petertodd

README.md

@@ -11,8 +11,8 @@ focus on providing tools to manipulate the internals of how Bitcoin works.
 
     sudo apt-get install libssl-dev
 
-The RPC interface, bitcoin.rpc, is designed to work with Bitcoin Core v0.11.0
-Older versions mostly work but there do exist some incompatibilities.
+The RPC interface, bitcoin.rpc, is designed to work with Bitcoin Core v0.13.0
+Older versions may work but there do exist some incompatibilities.
 
 
 ## Structure