Check cloneable by git

Case in using ghq.
gem-src many send requests to the hompage when execute `gem i bundler` but that is not url for git.
It takes many time. So I want to check it is git url before execute `git clone #{repository}`.

Author: sachin21

README.md

@@ -88,6 +88,7 @@ This requires ghq (https://github.com/motemen/ghq) to be installed, and either o
 
     1) `GEMSRC_USE_GHQ` environment variable
     2) add `gemsrc_use_ghq` configuration in your .gemrc
+    3) Install nmap command needed for checking that URL is cloneable by git
 
 The directory structure should be like below in this mode.
 

lib/rubygems_plugin.rb

@@ -121,6 +121,7 @@ def git_clone(repository)
       return if @tested_repositories.include? repository
       @tested_repositories << repository
       return if github?(repository) && !github_page_exists?(repository)
+      return unless `nmap -sT -p 9418 #{URI.parse(repository).host}` =~ /open/
 
       puts "gem-src: #{installer.spec.name} - Cloning from #{repository}..." if verbose?