Added default impl.version to build.properties

Author: Rob Harrop

build.properties

@@ -6,6 +6,7 @@ bundle.out=${build.out}/bundle
 bundle.symbolicName=com.rabbitmq.client
 bundlor.home=bundlor
 dist.out=${build.out}/dist
+impl.version=0.0.0
 javac.debug=true
 javac.out=${build.out}/classes
 javadoc.out=build/doc/api