gemspec: Explicitly list 0 executables

This gem exposes no executables, so this makes that clear to the reader.

un.gemspec

@@ -19,6 +19,6 @@ Gem::Specification.new do |spec|
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{\A(?:test|spec|features)/}) }
   end
   spec.bindir        = "exe"
-  spec.executables   = spec.files.grep(%r{\Aexe/}) { |f| File.basename(f) }
+  spec.executables   = []
   spec.require_paths = ["lib"]
 end