Remove activesupport dependency

Thanks for a great library! I suspect this pull request isn't ready for merging, but hopefully it leads to some discussion.

Whenever currently does a `require "active_support/all"`. This loads the full active support suite - including monkey patches to many core classes, custom `to_json` handling, runtime dependencies on minitest and i18n, logging support and a host of other functionality. In a rails app that's unavoidable, but we use whenever in some non-rails projects and the invasive nature of activesupport isn't ideal.

As an experiment, this removes the activesupport dependency. In most cases, I was able to replace calls to AS methods (like `String#blank?`, `String#squish`, `String#starts_with` and `Object#class_eval`) with plain ruby alternatives. These changes have no user visible impact.

The remaining changes deal with the absence of the `Fixnum#{minutes,hours,days,weeks,months}`. The alternative is slightly more verbose, but (imho) no less maintainable. Unfortunately this would be a user visible change and has the potential to break working schedules after an upgrade (but only for people using whenever outside rails).

Those using whenever with rails would be unaffected due to rails depending on activesupport anyway.

I also had to change the minitest dependency to get the tests running on MRI 2.1, which isn't particularly relevant to this branch. I'd be happy to rebase the branch and remove it if required.