configurable cache-time

[rubiii]

hi nando,

we would prefer to not expire the cache after 24h. instead we would like to add something like a job to expire the cache, along with checking for yanked gems, etc.

what do you think about allowing users to configure whether the cache should expire at all and maybe also how long the specs.4.8.gz should be cached?

https://github.com/fnando/rubygems_proxy/blob/master/rubygems_proxy.rb#L110