title: Jenkins Sonar plugin suddenly stops working
body: <p>Our Jenkins builds started failing overnight with the error:</p>
  
  <pre><code>[ERROR] Failed to execute goal org.codehaus.mojo:sonar-maven-plugin:2.7:sonar (default-cli) on project &lt;project&gt;: 
  Execution default-cli of goal org.codehaus.mojo:sonar-maven-plugin:2.7:sonar failed: 
  Unable to load the mojo 'sonar' in the plugin 'org.codehaus.mojo:sonar-maven-plugin:2.7' due to an API incompatibility:
  org.codehaus.plexus.component.repository.exception.ComponentLookupException: org/codehaus/mojo/sonar/SonarMojo :
  Unsupported major.minor version 51.0
  </code></pre>
  
  <p>The Jenkins server is still running Java 6, but we did not change anything.</p>
  
  <p>What happened and how do we fix it?</p>