add pry for debugging awesomeness

This will allow you to use `pry` for debugging everywhere we use vault-test-tools. Although, you may need to move vault-test-tools into the development group as well to get a handle on it.

Usage:

```ruby
def buggy_method
binding.pry      # you add this line
foo = 1 * 2 + something_else
end
```

There are lots of advantages to using this over `require 'debug'; debugger`, one of which being that it comes with syntax highlighting and stack frame exploration out of the box. And it seems to work more often.