Adds `letter_opener` for previewing emails
Merge pull request #36 from moneyadviceservice/use-letter-opener

Adds `letter_opener` for previewing emails
Upgrades to Ruby 2.2.0 🎅
Bumps rails to 4.2
Removes `serve_static_assets` deprecation
Removes `#deliver` deprecations

Ideally this would now make use of `ActiveJob` for `#deliver_later` but
we can deal with that another time.
Merge pull request #41 from moneyadviceservice/rails-4-2

Upgrades Rails to 4.2 😍
Merge pull request #40 from moneyadviceservice/ruby-2-2-0

Upgrades to Ruby 2.2.0 🎅
Adds validation feature
Simplifies `.authenticate`
Ensures uniqueness of FCA numbers for Principals

* Reworks factory callbacks to run on `build` and `create`
Adds validation per the story comments

* Some fields are 'approximately' valid
* Needed to clarify web address validation so left this out for now
Reworks to use SitePrism's `elements`