Idea: Make a single call to `gem install`

I was discussing this with @soveran earlier.

I thought this was only possible on RubyGems 2.0, but apparently it's always been there. The syntax did change for 2.0.

1.8.x:

```
$ gem install ohm -v 1.1.2 ohm-contrib -v 1.1.0
```

2.0:

```
$ gem install ohm:1.1.2 ohm-contrib:1.1.0
```

In my scientific benchmark, it's "a lot" faster.
