CoreFormulaRepository: move require statements to the top

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.

Library/Homebrew/core_formula_repository.rb

@@ -1,4 +1,6 @@
 require "tap"
+require "tap_migrations"
+require "formula_renames"
 
 # A specialized {Tap} class to mimic the core formula file system, which shares many
 # similarities with normal {Tap}.
@@ -64,13 +66,11 @@ class CoreFormulaRepository < Tap
 
   # @private
   def formula_renames
-    require "formula_renames"
     FORMULA_RENAMES
   end
 
   # @private
   def tap_migrations
-    require "tap_migrations"
     TAP_MIGRATIONS
   end