support deleting attachments - Redmine 3.3.0+ only

[alexeyOnGitHub]

I submitted a request to Redmine developers to support deleting attachments: http://www.redmine.org/issues/17879

[alexeyOnGitHub]

I see the redmine patch was merged 3 months ago (see http://www.redmine.org/issues/14828) and now we can add support to the java library

[alexeyOnGitHub]

done