Specifiy mixlib shellout 1.6.0 or greater

Using mixlib shellout 1.3.0 which was already installed on my box the rake build would fail with the following.  Looks like 'error?' didn't exist as a method until later.  When I installed mixlib shellout 1.6.0 the 'error?' method is available. 

   

rake aborted!
undefined method `error?' for #<Mixlib::ShellOut:0x007fb5249b2090>
/Users/derek.olsen/src/omnibus-flapjack/Rakefile:143:in `block in <top (required)>'
/usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/1.9.1/rake/task.rb:205:in `call'
/usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/1.9.1/rake/task.rb:205:in `block in execute'
/usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/1.9.1/rake/task.rb:200:in `each'
/usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/1.9.1/rake/task.rb:200:in `execute'