use new patch directory convention for ruby 2.1

As per request. Install works fine, but I find the output of rvm list known a bit confusing:

```
# MRI Rubies
[ruby-]1.8.6[-p420]
[ruby-]1.8.7[-p374]
[ruby-]1.9.1[-p431]
[ruby-]1.9.2[-p320]
[ruby-]1.9.3[-p484]
[ruby-]2.0.0-p195
[ruby-]2.0.0[-p353]
[ruby-]2.1.0
[ruby-]2.1.0-head
ruby-head
```

I think it should say `[ruby-]2.1-head`instead of `[ruby-]2.1.0-head`.