Automatically manage versions

[gapple]

Currently a code update is required to track a new minor version, so when the new minor version branch is created and issues automatically migrated there is a bit of a gap in the data.

Probably best to fetch info on available versions from the API (also see #19) before collecting stats.

Old minor releases still have a few issues assigned (e.g. 8.0 currently has 4 issues, versus ~2800 for 8.4), but are not supported. Tracking could be stopped on these branches once they are EOL.