Bump org.codehaus.mojo:exec-maven-plugin from 1.2.1 to 3.3.0 in /samp…

…les/concurrent-modification (#210)

Bumps [org.codehaus.mojo:exec-maven-plugin](https://github.com/mojohaus/exec-maven-plugin)
from 1.2.1 to 3.3.0.

samples/concurrent-modification/pom.xml

@@ -20,7 +20,7 @@
 			<plugin>
 				<groupId>org.codehaus.mojo</groupId>
 				<artifactId>exec-maven-plugin</artifactId>
-				<version>1.2.1</version>
+				<version>3.3.0</version>
 				<configuration>
 					<mainClass>io.dgraph.example.MultiThreadedMutationLauncher</mainClass>
 				</configuration>