Update maven compiler plugin to 3.1 to match sdk.

Author: Raghu Angadi

contrib/kafka/pom.xml

@@ -44,7 +44,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.2</version>
+        <version>3.1</version>
         <configuration>
           <source>1.7</source>
           <target>1.7</target>