Fix type for Stratify::ITunes::Activity#persistent_id

In an "iTunes Music Library.xml" file, Persistent IDs look like:

  <key>Persistent ID</key><string>5E392CBEF8E9ED1F</string>

Notice that the ID is *not* an integer. So, we shouldn't be storing it
as an integer.
Upgrade to Mongoid 3

  - Update Stratify::Activity and Stratify::Collector to comply with
    Mongoid DSL changes
  - Update mongoid.example.yml to use the format expected by Mongoid 3
  - Configure Mongoid 3 to use the same time zone behavior that was
    present in Mongoid 2 (i.e., `use_activesupport_time_zone: false`).
Update to latest version of mongoid_rails_migrations

Upgrade mongoid_rails_migrations for compatibility with Mongoid 3.
Update to latest version of rspec-rails (2.14.0)