Remove default_executable= from gemspec since it's deprecated

annotate.gemspec

@@ -10,7 +10,6 @@ Gem::Specification.new do |s|
   s.required_rubygems_version = Gem::Requirement.new(">= 0") if s.respond_to? :required_rubygems_version=
   s.authors = ["Cuong Tran", "Alex Chaffee", "Marcos Piccinini"]
   s.date = %q{2009-10-23}
-  s.default_executable = %q{annotate}
   s.description = %q{Annotates Rails Models, routes, fixtures, and others based on the database schema.}
   s.email = ["alex@stinky.com", "ctran@pragmaquest.com", "x@nofxx.com"]
   s.executables = ["annotate"]