Upgrades rubygems for 1.9.1: 1.3.5 -> 1.3.7

This upgrades RubyGems so that bundler can be installed on Ruby 1.9.1.

The bundler gem is dependent on RubyGems >= 1.3.6. Otherwise you get this:

```
$ gem install bundler
ERROR:  Error installing bundler:
	bundler requires RubyGems version >= 1.3.6
```