Bump exec-maven-plugin to 3.1.0

[INFO] --- exec-maven-plugin:1.6.0:exec (make-sure-validation-files-are-in-sync) @ hbase-shaded-with-hadoop-check-invariants ---
[ERROR] Command execution failed.
java.io.IOException: Stream closed
    at java.lang.ProcessBuilder$NullOutputStream.write (ProcessBuilder.java:433)
    at java.io.OutputStream.write (OutputStream.java:116)
    at java.io.BufferedOutputStream.flushBuffer (BufferedOutputStream.java:82)
    at java.io.BufferedOutputStream.flush (BufferedOutputStream.java:140)
    at java.io.FilterOutputStream.close (FilterOutputStream.java:158)
    at org.apache.commons.exec.DefaultExecutor.closeProcessStreams (DefaultExecutor.java:306)
    at org.apache.commons.exec.DefaultExecutor.executeInternal (DefaultExecutor.java:387)
    at org.apache.commons.exec.DefaultExecutor.execute (DefaultExecutor.java:166)
    at org.codehaus.mojo.exec.ExecMojo.executeCommandLine (ExecMojo.java:804)
    at org.codehaus.mojo.exec.ExecMojo.executeCommandLine (ExecMojo.java:751)
    at org.codehaus.mojo.exec.ExecMojo.execute (ExecMojo.java:313)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:190)
    at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:186)
    at java.util.concurrent.FutureTask.run (FutureTask.java:266)
    at java.util.concurrent.Executors$RunnableAdapter.call (Executors.java:511)
    at java.util.concurrent.FutureTask.run (FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:624)
    at java.lang.Thread.run (Thread.java:748)