Initialize config.firebolt in the railtie.
Fix old reference to config.frequency.
Write to the cache file is enabled, not readable.

When writing the cache set to the cache file, check
`config.cache_file_enabled?` instead of `config.use_file_warmer?` since
it checks if the file is readable, which will be false until the file is
created.
Set the defaults before grabbing the user config.
Bump version to 0.9.2.