requires ohai gem to do system detection

:heart: 

without requiring the ohai gem I get the following:
```
vagrant@pleaserun:/tmp/redis-2.8.6$ sudo pleaserun --install /tmp/redis/bin/redis-server
/var/lib/gems/1.9.1/gems/pleaserun-0.0.1/lib/pleaserun/detector.rb:29:in `detect': PleaseRun::Detector::UnknownSystem (PleaseRun::Detector::UnknownSystem)
	from /var/lib/gems/1.9.1/gems/pleaserun-0.0.1/lib/pleaserun/cli.rb:102:in `execute'
	from /var/lib/gems/1.9.1/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
	from /var/lib/gems/1.9.1/gems/clamp-0.6.3/lib/clamp/command.rb:125:in `run'
	from /var/lib/gems/1.9.1/gems/pleaserun-0.0.1/bin/pleaserun:7:in `<top (required)>'
	from /usr/local/bin/pleaserun:19:in `load'
	from /usr/local/bin/pleaserun:19:in `<main>'
vagrant@pleaserun:/tmp/redis-2.8.6$
```