Add to require whenever/version

[taketo1113]

This Pull Request adds to require "whenever/version".
With this change, requiring only whenever will allow Whenever::VERSION to be accessed from anywhere.

Expected behavior

require 'whenever'

Whenever::VERSION
#=> "1.0.0"

Actual behavior

require 'whenever'

Whenever::VERSION
(irb):3:in '<main>': uninitialized constant Whenever::VERSION (NameError)