fix show-stack -c (slop error?)

```
[68] (pry) #<PryDebugger::Processor>: 0> show-stack -c
TypeError: true can't be coerced into Fixnum
from /Users/john/.rvm/gems/ruby-1.9.3-p194/gems/pry-stack_explorer-0.4.5/lib/pry-stack_explorer/commands.rb:278:in `-'
[69] (pry) #<PryDebugger::Processor>: 0> 
```
