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: Ic1742e4fd2aded5c05ee98a2496d9d00224a5f1e

pom.xml

diff --git a/pom.xml b/pom.xml
index ee00346..2bf4363 100644
--- a/pom.xml
+++ b/pom.xml
@@ -89,6 +89,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-shade-plugin</artifactId>
+        <version>2.3</version>
         <configuration>
           <promoteTransitiveDependencies>true</promoteTransitiveDependencies>
           <artifactSet>