Replace overwrite_shebang.rb with --env-shebang when installing rubygems.

[jfmyers9]

We added a fix which overwrites the shebang of the default gems with #!/usr/bin/env ruby.

As of rubygems/rubygems#2271, the --env-shebang should be present on the setup command. We should use that and remove the overwrite_shebang.rb scripts when it is released in a future version.