Update gemspec to match Gemfile

[steveyken]

This brings parity when fat_free_crm is loaded as an engine, via a gem.

Fixes following bundler error when including fat_free_crm as a gem

Fetching https://github.com/fatfreecrm/acts_as_commentable.git
Fetching https://github.com/CloCkWeRX/responds_to_parent.git
Fetching https://github.com/fatfreecrm/fat_free_crm.git
Fetching gem metadata from http://rubygems.org/...........
Resolving dependencies...
Could not find compatible versions

Because acts_as_commentable >= 6.0.0, < 6.1.A could not be found in https://github.com/fatfreecrm/acts_as_commentable.git (at rails-61@e695ef5)
  and every version of fat_free_crm depends on acts_as_commentable ~> 6.0.0,
  fat_free_crm cannot be used.
So, because Gemfile depends on fat_free_crm >= 0,
  version solving has failed.