Update xrpl version to fix standalone build (#56)

* The rest of the code was updated in #54 (89ab7e5) to allow for
  a combined build for the next version of rippled. Unfortunately, this
  broke the standalone build. Point conan at the latest beta so the
  build can work.
* This will need to be updated again after rippled 2.3 is released.
* Resolves #55

conanfile.txt

@@ -1,5 +1,5 @@
 [requires]
-xrpl/2.1.0
+xrpl/2.3.0-b4
 
 [generators]
 CMakeDeps