resolution can be nil in persist_resolution if dependencies were not resolved

If none of the dependencies can be resolved the resolution object is nil in persist_resolution. Check and print the "Could not resolve the dependencies." message

    /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/action/persist_resolution_mixin.rb:11:in `persist_resolution': undefined method `correct?' for nil:NilClass (NoMethodError)
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/action/resolve.rb:27:in `run'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:169:in `resolve!'
	from /Users/csanchez/dev/librarian-puppet/lib/librarian/puppet/cli.rb:63:in `install'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:26:in `block (2 levels) in bin!'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:31:in `returning_status'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:26:in `block in bin!'
	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:47:in `with_environment'

	from /Users/csanchez/.rvm/gems/ruby-1.9.3-p327/gems/librarian-0.1.0/lib/librarian/cli.rb:26:in `bin!'
	from ./bin/librarian-puppet:7:in `<main>'