Clean up rake tasks

* Remove metric_fu tasks
  - metric_fu now has its own runner
  - metric_fu is not part of the bundle, so these tasks were not
    available when run with `bundle exec`
  - loading these tasks takes an insanely long time
* Delay loading yaml dependency until needed
* Delay requiring Reek sniffer until needed
* Remove dependency declarations for obsolete tasks
* Remove unused Rakefile constants
* Remove needless dependencies
* Do not manipulate the load path: The 'lib' directory is already added
  to the load path by 'bundler/gem_tasks'.
* Remove needless square brackets
* Expect config/ to exist: its contents is in git, and it should not be
  removed by `rake clobber`.