Update rpc version to 5.3.1 (#30)

* add thread-pool-ref xsd attribute

* readme format

* update rpc starter version to 5.3.1

Author: NeGnail

sofaboot-dependencies/pom.xml

@@ -40,7 +40,7 @@
         <spring.boot.version>1.4.2.RELEASE</spring.boot.version>
         <sofa.boot.version>2.3.1</sofa.boot.version>
         <sofa.ark.version>0.2.0</sofa.ark.version>
-        <rpc.starter.version>5.3.0</rpc.starter.version>
+        <rpc.starter.version>5.3.1</rpc.starter.version>
         <!-- sofa-common-tools for log -->
         <sofa.common.tools.version>1.0.12</sofa.common.tools.version>
         <bolt.version>1.4.1</bolt.version>