Spring cleaning: remove user records where github_id is nil (related to #979)

For real github accounts, the github_id is not nil. So if the id is nil, it is a fake account or an input error.
Updated seeds with github_id

Author: emcoding

db/migrate/20180524190818_destroy_non_existing_git_hub_users.rb

@@ -0,0 +1,5 @@
+class DestroyNonExistingGitHubUsers < ActiveRecord::Migration[5.1]
+  def up
+    execute 'DELETE FROM users WHERE github_id IS NULL'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180514195656) do
+ActiveRecord::Schema.define(version: 20180524190818) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

spec/factories/users.rb

@@ -1,6 +1,7 @@
 FactoryBot.define do
   factory :user, aliases: [:member] do
     github_handle { FFaker::InternetSE.unique.user_name_variant_short }
+    sequence(:github_id, 123)
     name     { FFaker::Name.name }
     email    { FFaker::Internet.email }
     location { FFaker::Address.city }