Ability to push individual tags or refs

Hi Andrew,

This pull request enables the plugin user to [push names or specs](http://download.eclipse.org/jgit/docs/jgit-2.0.0.201206130900-r/apidocs/org/eclipse/jgit/api/PushCommand.html#add(java.lang.String\)) to a remote Git repository. Before you only could push everything. Here's an example on how to push just a single tag:

    task pushTag(type: GitPush) {
        namesOrSpecs = ['v1.15']
    }

Unfortunately, there was no test that I could add on. My guess is that it was kind of hard to test this as it requires a dedicated remote repository to push to. Please let me know if you want me to put in some extra work.

Here're the changes of this pull request on a high-level. Really nothing too dramatic:

* Added property for pushing individual tags or refs. 
* Upgrade to Wrapper 1.6. 
* Fixed deprecation warnings.