Fix gemspec

Reference:
https://github.com/tiredpixel/sidekiq-spy/commit/16ead977fa837966736b7dc50b3cfa6a3dd7d3c2

ext/mkrf_conf.rb

@@ -0,0 +1,9 @@
+require 'rubygems/dependency_installer'
+
+if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.1.0')
+  Gem::DependencyInstaller.new.install 'curses', '~> 1.0'
+end
+
+File.open(File.expand_path('../Rakefile', __FILE__), 'w') do |f|
+  f.puts 'task :default'
+end