Synchronize generated gemspec with current bundler template

See template at https://github.com/rubygems/rubygems/blob/d786a1ccca3922d21572f77b021c51bfaf005819/bundler/lib/bundler/templates/newgem/newgem.gemspec.tt#L34

Additionally, exclude .gem files, since there seem to be several gems built from swagger-codegen that are recursively including older .gem versions, and thus growing in size exponentially

modules/swagger-codegen/src/main/resources/ruby/gemspec.mustache

@@ -36,8 +36,14 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'autotest-growl', '~> 0.2', '>= 0.2.16'
   s.add_development_dependency 'autotest-fsevent', '~> 0.2', '>= 0.2.12'
 
-  s.files         = `find *`.split("\n").uniq.sort.select { |f| !f.empty? }
-  s.test_files    = `find spec/*`.split("\n")
+  gemspec = File.basename(__FILE__)
+  spec.files = IO.popen(%w[find * -print0], chdir: __dir__, err: IO::NULL) do |ls|
+    ls.readlines("\x0", chomp: true).reject do |f|
+      (f == gemspec) ||
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git Gemfile]) ||
+        f.end_with?(".gem")
+    end
+  end
   s.executables   = []
   s.require_paths = ["lib"]
 end