Fix `ON DELETE CASCADE` migrations.

Incorrect syntax.

db/migrate/20160824124900_add_table_issue_metrics.rb

@@ -25,7 +25,7 @@ class AddTableIssueMetrics < ActiveRecord::Migration
 
   def change
     create_table :issue_metrics do |t|
-      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, dependent: :delete, null: false
+      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: { on_delete: :cascade }, null: false
 
       t.datetime 'first_mentioned_in_commit_at'
       t.datetime 'first_associated_with_milestone_at'

db/migrate/20160825052008_add_table_merge_request_metrics.rb

@@ -25,7 +25,7 @@ class AddTableMergeRequestMetrics < ActiveRecord::Migration
 
   def change
     create_table :merge_request_metrics do |t|
-      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: true, dependent: :delete, null: false
+      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: { on_delete: :cascade }, null: false
 
       t.datetime 'latest_build_started_at'
       t.datetime 'latest_build_finished_at'

db/migrate/20160915042921_create_merge_requests_closing_issues.rb

@@ -25,8 +25,8 @@ class CreateMergeRequestsClosingIssues < ActiveRecord::Migration
 
   def change
     create_table :merge_requests_closing_issues do |t|
-      t.references :merge_request, foreign_key: true, index: true, dependent: :delete, null: false
-      t.references :issue, foreign_key: true, index: true, dependent: :delete, null: false
+      t.references :merge_request, foreign_key: { on_delete: :cascade }, index: true, null: false
+      t.references :issue, foreign_key: { on_delete: :cascade }, index: true, null: false
 
       t.timestamps null: false
     end