Add setup/config for Sentry error reporting

We're using Sentry for error capture here at nested.  Would be good to
track errors like our other in-house applications.

Gemfile

@@ -1,27 +1,23 @@
 source 'https://rubygems.org'
 
-gem 'rake'
+gem 'airbrake', require: false
 gem 'dotenv'
-
-gem 'sequel'
-gem 'mysql2', platform: :ruby
-gem 'pg', platform: :ruby
+gem 'erubis'
 gem 'jdbc-mysql', platform: :jruby
 gem 'jdbc-postgres', platform: :jruby
-
-gem 'sinatra'
-gem 'rack-flash3'
-gem 'erubis'
-gem 'macmillan-utils'
 gem 'lru_redux'
-
+gem 'macmillan-utils'
+gem 'mysql2', platform: :ruby
+gem 'nokogiri', '~> 1.8.2'
+gem 'pg', platform: :ruby
 gem 'puma', require: false
-
-gem 'airbrake', require: false
-gem 'statsd-ruby', require: false
 gem 'rack-cors'
-
-gem 'nokogiri', '~> 1.8.2'
+gem 'rack-flash3'
+gem 'rake'
+gem 'sentry-raven', require: false
+gem 'sequel'
+gem 'sinatra'
+gem 'statsd-ruby', require: false
 
 group :development do
   gem 'shotgun'

Gemfile.lock

@@ -32,6 +32,8 @@ GEM
     docile (1.1.5)
     dotenv (2.2.1)
     erubis (2.7.0)
+    faraday (0.15.2)
+      multipart-post (>= 1.2, < 3)
     ffi (1.9.21)
     ffi (1.9.21-java)
     formatador (0.2.5)
@@ -79,6 +81,7 @@ GEM
     mini_portile2 (2.3.0)
     minitest (5.11.3)
     multi_test (0.1.2)
+    multipart-post (2.0.0)
     mustermann (1.0.2)
     mysql2 (0.4.10)
     nenv (0.3.0)
@@ -143,6 +146,8 @@ GEM
     ruby-progressbar (1.9.0)
     ruby_dep (1.5.0)
     safe_yaml (1.0.4)
+    sentry-raven (2.7.4)
+      faraday (>= 0.7.6, < 1.0)
     sequel (5.5.0)
     shellany (0.0.1)
     shotgun (0.9.2)
@@ -211,6 +216,7 @@ DEPENDENCIES
   rake
   rspec
   rubocop
+  sentry-raven
   sequel
   shotgun
   sinatra

config.ru

@@ -19,6 +19,19 @@ if ENV['AIRBRAKE_API_KEY'] && ENV['AIRBRAKE_PROJECT_ID']
   end
 end
 
+if ENV['SENTRY_DSN']
+  require 'raven'
+
+  Raven.configure do |config|
+    config.dsn                 = ENV['SENTRY_DSN']
+    config.current_environment = ENV.fetch('RACK_ENV', 'development')
+    config.environments        = ['production']
+    config.logger              = Bandiera.logger
+  end
+
+  use Raven::Rack
+end
+
 if ENV['RACK_CORS_ORIGINS']
   require 'rack/cors'