Remove default_formula 
Remove default_formula 

Homebrew has removed use of the default_formula attribute/method in Requirement. 
Continue using Requirement to detect need for system/brewed gcc, but code formula
explicitly when needed.
SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael,SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael