Update GitlabRepository.getProjectForPath() to no longer require an admin user

In our usage of this plug-in, we'd like to restrict the bot user to the most restricted access possible.
* Updated GitlabRepository.getProjectForPath() to use the non-admin [/projects](http://doc.gitlab.com/ce/api/projects.html) endpoint via GitlabAPI.getProjects(). This allows the use of a user with just the [Reporter](http://doc.gitlab.com/ce/permissions/permissions.html) role.
* Updated the Jenkins version in pom.xml.
* Updated README.md.