Ensure pry compatibility for Ruby 1.8

:information_desk_person: This should fix the [build error](https://travis-ci.org/capistrano/capistrano/builds/30553023) on the `legacy-v2` branch. :blush: 

* Pry has moved to using Ruby 1.9 hash syntax from v0.10, which causes syntax errors when using Ruby 1.8.
  Locking the version of pry to a compatible version prevents this.