This commit will help to mitigate bug Homebrew/homebrew#42553 on certain old Homebrew installations
(e.g. `osx_image: xcode6.4` on Travis CI) for future core/formula
separation.
On that particular Homebrew installations, `formulary.rb` will be loaded after
`git pull` is finished during `brew update`, which will then load `core_formular_repository.rb`.
By introducing `require "tap_migrations"` and `require "formula_renames"` at the top of
`core_formular_repository.rb`, we could use `tap_migrations.rb` as a
vector to preform certain hack for future core/formula separation.
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