use "agile" endpoint over now removed "greenhopper"

[adehad]

The "greenhopper" endpoint is deprecated in Jira version ~6, our test instances don't support those endpoints any more and it is causing troubles to our users: #1146 and also with ranking #1160 .

Some details about deprecation can be found here:
https://community.atlassian.com/t5/Jira-Core-Server-questions/Greenhooper-REST-API/qaq-p/816504

https://web.archive.org/web/20131022032352/http://blogs.atlassian.com/2013/08/renaming-greenhopper-and-bonfire/

We also have this documented in our code:

jira/jira/resources.py

Lines 1187 to 1192 in fa5dea1

	GREENHOPPER_REST_PATH = "greenhopper"
	""" Old, private API. Deprecated and will be removed from Jira on the 1st February 2016. """
	AGILE_EXPERIMENTAL_REST_PATH = "greenhopper/experimental-api"
	""" Experimental API available in Jira Agile 6.7.3 - 6.7.6, basically the same as Public API """
	AGILE_BASE_REST_PATH = "agile"
	""" Public API introduced in Jira Agile 6.7.7. """

[adehad]

@studioj As it has been about 5 years since this endpoint's removal (renaming technically) and the package aims not to support older versions (especially ones we can't even test for) I think we should be happy with this change.

If users do want to use the older "greenhopper" version, they can still pass the endpoint manually as part of the options OR just use an older version of this package.

[studioj]

in the assumption we do test agile stuff, good to go