Initial update to the JIRA v6 API (fixes #6, aptly numbered).

Some changes were inspired by PR #9 which was closed by author @treymarc.

This disables the ApprovalFunctionTest, because junit was failing
due to ClassNotFoundException looking for UserKeyStore. This will likely
require implementing some kind of compatibility layer due to the User
API changes between JIRA 5 and 6.

Some plugins and dependencies have also been updated to their latest versions.