Only bootstrap buildout if missing or outdated

Parse `bin/buildout annotate` to get the wanted buildout version and
`bin/buildout --version` for the currently used buildout version.

`Annotate` output seems to be stable over buildout releases, whereas
`--version` has just been introduced in 2.1.0. This doesn't matter
though as this check is being introduced together with the upgrade to
buildout 2.1, thus from now on this information is available.

Fixes hhucn/adhocracy.hhu_theme#265.