Move loading of 'git' library nearer to where it's used.

This is necessary because `require "git"` (at time of writing) will
error if the `git` program is not available.

Upstream (ruby git) issue:
ruby-git/ruby-git#311

Related fpm issues: #1748, #1751

lib/fpm/package/gem.rb

@@ -4,7 +4,6 @@
 require "fileutils"
 require "fpm/util"
 require "yaml"
-require "git"
 
 # A rubygems package.
 #
@@ -105,6 +104,7 @@ def download(gem_name, gem_version=nil)
     FileUtils.mkdir(download_dir) unless File.directory?(download_dir)
 
     if attributes[:gem_git_repo]
+      require "git"
       logger.debug("Git cloning in directory #{download_dir}")
       g = Git.clone(attributes[:gem_git_repo],gem_name,:path => download_dir)
       if attributes[:gem_git_branch]