Limit matters to those soon/recently

[techieshark]

• the prev code used a hard code fixed date, which became stale
• this calculates date dynamically
• Fixes #150

The following should be run after this pushing this code to heroku:

heroku run rake legistar_matters:drop 
heroku run rake legistar_matters:load 
heroku run rake legistar_attachments:drop
heroku run rake legistar_attachments:load

Alternatively you can run a complete refresh of the Legistar content: heroku run rake legistar_all:refresh.