Switch emails (again).
Add the json gem as a dependency.
Skip initialization/warming via an environment variable.

Setting FIREBOLT_SKIP_WARMING now skips initialization of SuckerPunch &
Rufus and the initial warming. They're also skipped if RAILS_ENV is set
to 'test'.
Rename `frequency` config option to `warming_frequency`.
Add Config#use_file_warmer?

Config#use_file_warmer? checks if cache file is enabled and the
readable.
Bumping to v1.0.0.alpha.