Fix Ticket 7623 getUser API CALL change in JIRA REST API

[Copro]

Fix required because if JIRA API change

[fmancardi]

IMHO this fix can be done only if is compatible with older JIRA versions, if not best option is to check for JIRA version and use call specific to each version

[Copro]

I thought about that and checked also older JIRA API releases. In my option this call has not been working from the beginning (I also created a pull request in the original project which has been inactive since 2013: FayP/JIRA-php#9)
15-Feb-2012 - JIRA API 5.0 : https://docs.atlassian.com/jira/REST/5.0/#id199293 (OK)
21-May-2013 - JIRA API 6.0 : https://docs.atlassian.com/jira/REST/6.0/#id160233 (OK)
29-Jun-2015 - JIRA API 7.0-SNAPSHOT: https://docs.atlassian.com/jira/REST/7.0-SNAPSHOT/#api/2/user-getUser (OK)

--> So there should be no need to check for the JIRA Version - the function getUser was implemented wrong in the first place.