For users whose local brew is at around 2015-06-11 to 2015-08-06,
running `brew update` will emit following error:
Error: uninitialized constant Formulary::CoreFormulaRepository
This is caused by the same bug described in Homebrew/homebrew#42553.
This commit workarounds this issue and restores `brew update` compatibility
for users mentioned above.
Also cleanup legacy `require "cmd/tap"`.