Add Pry as a development dependency

[Pry](http://pryrepl.org/) is a Ruby shell and is very useful for
debugging.

You can use Pry by specifying `RUBYOPT='-r pry'` when running the
vCloud Tools CLI commands and by adding breakpoints in your code, for
example:

```
binding.pry
```

Hitting ^D will exit the shell and resume code execution after the break
point.
Merge pull request #64 from alphagov/71926812-add-pry-dependency

[#71926812] Add Pry as a development dependency