rbinstall.rb: fix .gemspec permissions

* tool/rbinstall.rb (gem): Fix permissions of bundled gems
  specification files.  [ruby-core:65700] [Bug #10383]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47916 b2dd03c8-39d4-4d8f-98ff-823fe69b080e