Don't save settings values at module load time

Previously, we'd "cache" the RATELIMIT_ENABLE and CACHE_PREFIX settings
at module load time. That makes it harder to create tests that change
these values.

This fixes that by changing the code so we're not caching the settings
values.

Fixes #38
Merge pull request #39 from willkg/fix-settings

Don't save settings values at module load time