Invoke jenkins-cli.jar as jenkins_user so that it's private key is used for authentication

Otherwise it's only working with default (unsecured) Jenkins installations
Merge pull request #20 from andreychernih/install-plugins-with-jenkins-user

Invoke jenkins-cli.jar as jenkins_user so that it's private key is used for authentication
Run cli from jenkins user
Update contributors
Bump version: 3.0.0 → 3.1.0