GitHub response 403 doesn't have ratelimit reset time

[Zlopez]

Currently we are expecting that GitHub response will always contain reset time for ratelimit, which isn't true when GitHub returns 403.

There are several options to deal with this:

• Save reset time to somewhere where we could retrieve it later (global variable in anitya.lib.backends.github looks like quick solution)
• Don't save it, check_service has it anyway, so it will know when to check the projects again, just raise RateLimitException without time.