Remove default argument from Homebrew::InstallationError

This class is never instantiated directly, and all subclasses
initializers have different signatures and do not rely on this.

Library/Homebrew/exceptions.rb

@@ -83,7 +83,7 @@ module Homebrew
   class InstallationError < RuntimeError
     attr_reader :formula
 
-    def initialize formula, message=""
+    def initialize(formula, message)
       super message
       @formula = formula
     end