The merge of the PR772 (https://github.com/jenkinsci/job-dsl-plugin/pull/772) break the build.

See https://jenkins.ci.cloudbees.com/job/plugins/job/job-dsl-plugin/1915/console

```
:job-dsl-plugin:compileTestGroovywarning: [options] bootstrap class path not set in conjunction with -source 1.6
/scratch/jenkins/workspace/plugins/job-dsl-plugin/job-dsl-plugin/build/tmp/compileTestGroovy/groovy-java-stubs/javaposse/jobdsl/plugin/DslEnvironmentImplSpec.java:40: error: TestAbstractExtensibleContext is not abstract and does not override abstract method setMetaClass(MetaClass) in GroovyObject
public static class TestAbstractExtensibleContext
^
1 error
1 warning
startup failed:
```