Revert "Call context-specific includes for each taggable_on call"

This reverts commit e44b69d6e378c4c5196612d30c72996f7216eabf.

This undid the work of 16c45677d2b4eb81 such that we were now
re-including most the library on subsequent calls of the macro method
like:

    acts_as_taggable_on :pros
    acts_as_taggable_on :cons

The change was made in order to fix dirty tracking on #321, but the
proper fix is that the dirty tracking module is missing a hook
`acts_as_taggable_on` macro that calls super.
Fix dirty tracking without repeat module includes

References #339
Related module preserves ordered tags option

Following b9b2747fbe, a spec broke for related when ordered tags are in
use, since the hook for repeated calls of the macro was not maintaining
the preserve order value.

This hook pattern likely needs to be updated throughout the library now
that the `acts_as_ordered_taggable_on` exists as an alternative macro,
but I'm just fixing what broke a spec within the scope of my current
branch.