Use POST instead of GET for JQL searches to get around 50 item limit in OnDemand

We are using Atlassian OnDemand which apparently configures the JIRA REST API to a maxCount per page of 50. Since the jql query does not seem to iterate over pages I've changed the request to POST with an explicit max items of 1000.

I've also adjusted the Rakefile to work with Rake 10.