Include all "supported" databases in our bundle

It's not really nice to force people to change Gemfile which is
under SCM control. This also enables us to make the database
adapter configurable.

Gemfile

@@ -24,10 +24,12 @@ gem 'rails_12factor', group: :production
 # http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#responders
 gem 'responders', '~> 2.0'
 
-# as the database for Active Record
-# choose only one
+# as supported databases
+# WARNING: please refrain from using sqlite for anything serious as it is not
+#          performant enough
 gem 'mysql2'
-# gem 'pg'
+gem 'pg'
+gem 'sqlite3'
 
 # for tracking data changes
 gem 'paper_trail'
@@ -239,8 +241,6 @@ group :development do
   # for static code analisys
   gem 'rubocop', require: false
   gem 'rubocop-rspec'
-  # as database
-  gem 'sqlite3'
   # to open mails
   gem 'letter_opener'
   # to open mails in browser

Gemfile.lock

@@ -342,6 +342,7 @@ GEM
     parser (2.5.1.2)
       ast (~> 2.4.0)
     pdf-core (0.2.5)
+    pg (1.1.2)
     phantomjs (2.1.1.0)
     piwik_analytics (1.0.2)
       actionpack
@@ -653,6 +654,7 @@ DEPENDENCIES
   omniauth-google-oauth2
   omniauth-openid
   paper_trail
+  pg
   phantomjs
   piwik_analytics (~> 1.0.1)
   poltergeist