[FIXED JENKINS-21274] Fixed startup failure when maven-plugin is disabled, compiled with JDK 1.6. SEE https://wiki.jenkins-ci.org/display/JENKINS/Tips+for+optional+dependencies for details.
Merge pull request #30 from ikedam/feature/JENKINS-21274_UnexpectedClassLoading

[JENKINS-21274] Fixed startup failure without maven-plugin when compiled with JDK 1.6