Add signing key stuff to gemspec, so I don't forget it

(sorry for the noise to those running edge capy on bundler)

capybara.gemspec

@@ -36,4 +36,11 @@ Gem::Specification.new do |s|
   s.add_development_dependency("cucumber", [">= 0.10.5"])
   s.add_development_dependency("rake")
   s.add_development_dependency("pry")
+
+  if File.exist?("gem-private_key.pem")
+    s.signing_key = 'gem-private_key.pem'
+  else
+    puts "WARNING!! No private key specified, gem will not be signed!"
+  end
+  s.cert_chain = ['gem-public_cert.pem']
 end