title: org/sonar/api/utils/SonarException : Unsupported major.minor version 51.0
body: <p>after upgrading from 3.3.2 to Sonarqube 5.1.2, my maven build for a project that needs to be built with jdk 1.6 is failing with:</p>
  
  <pre><code>build   03-Dec-2015 19:33:10    [INFO] ------------------------------------------------------------------------
  build   03-Dec-2015 19:33:10    [ERROR] Failed to execute goal org.codehaus.mojo:sonar-maven-plugin:2.6:sonar (default-cli) on project my_project: Execution default-cli of goal org.codehaus.mojo:sonar-maven-plugin:2.6:sonar failed: An API incompatibility was encountered while executing org.codehaus.mojo:sonar-maven-plugin:2.6:sonar: java.lang.UnsupportedClassVersionError: org/sonar/api/utils/SonarException : Unsupported major.minor version 51.0
  build   03-Dec-2015 19:33:10    [ERROR] -----------------------------------------------------
  build   03-Dec-2015 19:33:10    [ERROR] realm =    plugin&gt;org.codehaus.mojo:sonar-maven-plugin:2.6
  build   03-Dec-2015 19:33:10    [ERROR] strategy = org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy
  build   03-Dec-2015 19:33:10    [ERROR] urls[0] = file:/tmp/org/codehaus/mojo/sonar-maven-plugin/2.6/sonar-maven-plugin-2.6.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[1] = file:/tmp/org/apache/maven/shared/maven-dependency-tree/2.1/maven-dependency-tree-2.1.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[2] = file:/tmp/backport-util-concurrent/backport-util-concurrent/3.1/backport-util-concurrent-3.1.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[3] = file:/tmp/org/codehaus/plexus/plexus-interpolation/1.11/plexus-interpolation-1.11.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[4] = file:/tmp/org/codehaus/plexus/plexus-component-annotations/1.5.5/plexus-component-annotations-1.5.5.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[5] = file:/tmp/org/eclipse/aether/aether-util/0.9.0.M2/aether-util-0.9.0.M2.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[6] = file:/tmp/org/sonatype/plexus/plexus-sec-dispatcher/1.4/plexus-sec-dispatcher-1.4.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[7] = file:/tmp/org/codehaus/plexus/plexus-utils/1.5.5/plexus-utils-1.5.5.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[8] = file:/tmp/org/sonatype/plexus/plexus-cipher/1.4/plexus-cipher-1.4.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[9] = file:/tmp/org/codehaus/sonar/runner/sonar-runner-api/2.4/sonar-runner-api-2.4.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[10] = file:/tmp/commons-lang/commons-lang/2.6/commons-lang-2.6.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[11] = file:/tmp/com/google/guava/guava/10.0.1/guava-10.0.1.jar
  build   03-Dec-2015 19:33:10    [ERROR] urls[12] = file:/tmp/com/google/code/findbugs/jsr305/2.0.3/jsr305-2.0.3.jar
  build   03-Dec-2015 19:33:10    [ERROR] Number of foreign imports: 1
  build   03-Dec-2015 19:33:10    [ERROR] import: Entry[import  from realm ClassRealm[maven.api, parent: null]]
  build   03-Dec-2015 19:33:10    [ERROR] 
  build   03-Dec-2015 19:33:10    [ERROR] -----------------------------------------------------
  build   03-Dec-2015 19:33:10    [ERROR] -&gt; [Help 1]
  </code></pre>
  
  <p>seems like a compatibility issue, any ideas? 
  Pointing to the old sonar it still works fine, pointing to the new one, I get this.</p>
  
  <p>Thanks,
  Roberto</p>