Fixed `NoMethodError` on `pt show`.

When listing stories without the `all` argument a `NoMethodError` is thrown.

Trace when running `pt show`:
```
/usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/data_row.rb:10:in `initialize': undefined method `current_state' for #<PivotalTracker::Membership:0x007ff6931af310> (NoMethodError)
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/data_table.rb:10:in `new'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/data_table.rb:10:in `block in initialize'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/data_table.rb:10:in `map'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/data_table.rb:10:in `initialize'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/ui.rb:66:in `new'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/ui.rb:66:in `list'
	from /usr/local/lib/ruby/gems/2.1.0/gems/pt-0.7.2/lib/pt/ui.rb:20:in `initialize'
	from /usr/local/Cellar/ruby/2.1.1/lib/ruby/gems/2.1.0/gems/pt-0.7.2/bin/pt:10:in `new'
	from /usr/local/Cellar/ruby/2.1.1/lib/ruby/gems/2.1.0/gems/pt-0.7.2/bin/pt:10:in `<top (required)>'
	from /usr/local/Cellar/ruby/2.1.1/bin/pt:23:in `load'
	from /usr/local/Cellar/ruby/2.1.1/bin/pt:23:in `<main>'
```

I'm not a Ruby hacker so the commit is more to point to the problem.

Running Ruby on OS X 10.9.2: `ruby --version` ⟶ `ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin13.0]`