Joule grader will be released as part of whatever version of Core Moodle HQ accepts it in. Right now the earliest version Joule grader would be considered for inclusion would be Moodle 2.7. Moodle does not back port new features to older versions of Moodle. Joule grader would be considered a new feature. Joule Grader is a complex system and could have significant impacts to grading in at least one core activity, with additions from advanced forums to core forums probably 2 activities. We do not want to rush inclusion, instead we want to get it in right.
Moodlerooms doesn't plan to release Joule grader outside of a core release. Because it has such a significant impact on activity grading we do not want to have community members installing it incase Moodle HQ determines that apis need to change in order to provide the best functionality for the entire community. Moodlerooms is staffed to handle api changes internally, but we aren't staffed to fully support community members converting from Joule grader to the Moodle activity grader.
We really do need a different name for Joule grader when it is included into core.