Use timestamp from last commit for styles

[zuphilip]

Currently the updated fields shows what is hard coded in the csl, e.g. APA style. However, this is normally not equal to the last time the style was updated. In the zotero directory the timestamp from the last commit is used instead, see https://github.com/zotero/styles-repo/blob/master/scripts/generate-index#L220 . This also makes sense here in the citationstyles-repo.

[adam3smith]

styles-distribution updates timestamps. We should be able to just move the submodule to that. I'll do that in the next style update