Add version for Maven's shade plugin

Maven comlained that:

  [WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-shade-plugin is missing. @ line 86, column 15

Hence, we now add the version information.

Change-Id: I94d1a08ed9ba0a11807b76562590c4c22c807f79
diff --git a/pom.xml b/pom.xml
index 0c13237..515a107 100644
--- a/pom.xml
+++ b/pom.xml
@@ -290,6 +290,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-shade-plugin</artifactId>
+        <version>2.3</version>
         <configuration>
           <promoteTransitiveDependencies>true</promoteTransitiveDependencies>
           <artifactSet>