Defining a method that has the same name as a command breaks the command

I tested this on the `ls` command. Broken.

```
[1] pry(main)> def ls
             |   :wqeqw
             | end  
=> :ls
[2] pry(main)> ls
NoMethodError: undefined method `ceiling' for :wqeqw:Symbol
from /home/kyrylo/.gem/ruby/2.2.0/gems/pry-0.10.1/lib/pry/commands/ls/methods.rb:50:in `below_ceiling'
```

However, if you do the same with, say, the `whereami` command, it works well.
