Fix Rake task to add "spec" to loadpath

Putting spec helpers into `lib` is a bad idea so we need helpers to be
found elsewhere.
Add pry as development dependency

Not required to run but useless when debugging behaviour.
Add spec_helper to DRY up requires

Starting to duplicate which gets worse when adding new model specs.
Merge pull request #3 from brightbox/spec_helper

Add a spec helper to DRY up requires