Change 'Computed at' to time of DB export

[Daniel V. Egdal]

When you navigate to one of the statistics, the page mentions when the computation was done. However, this is not super useful when the developer DB doesn't update every time a computation is done. The developer update happens every ~3 days (I think), so instead the page should display the time of the latest DB export used. I found out sort of how to fix it, but I do not know ruby well enough to do a pull request for it. Here are my observations:

After getting the file here https://github.com/jonatanklosko/wca_statistics/blob/master/bin/update_database.rb#L18, run the unix command tail -1 wca-developer-database-dump.sql |grep -o '...................$' and save that.
Change this line https://github.com/jonatanklosko/wca_statistics/blob/master/core/statistic.rb#L25 to use the output from above.

[Jonatan Kłosko]

Hey @DanielEgdal! Good call, implemented it in 49618b7 :)