Default to using JIRA API v2

Avoids https://github.com/github/hubot-scripts/issues/1088 for the majority of new users (who should be on 5.0 or higher, seeing as it's been out for almost 2 years and Atlassian is now gearing up to release 6.1).
Merge pull request #1135 from gregmac/patch-6

Default to using JIRA API v2