Add the Git plugin as a dependency

[yahavi]

• [x] This pull request is created in the jfrog/jenkins-jfrog-plugin repository.

Resolve the following issue:

1. Add clone stage:

   stage('Clone') {
   steps {
       git branch: 'master', url: "https://github.com/jfrog/project-examples.git"
   }
   }

2. Receive the following error:

   org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
   WorkflowScript: 9: Invalid parameter “branch”, did you mean “name”? @ line 9, column 21.
                  git branch: ‘master’, url: “https://github.com/jfrog/project-examples.git”

The examples in https://github.com/jfrog/jenkins-jfrog-plugin/pull/18 may not work without this.