Prevent NULL values on ci_variables columns

To be more specific; key, and project_id. If either of these is
NULL its wrong, because:

1. storing a key value pair without key is just plain wrong
2. all variables should belong to a project

Non of these cases existed on the staging database.

db/post_migrate/20170508153950_add_not_null_contraints_to_ci_variables.rb

+class AddNotNullContraintsToCiVariables < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    change_column(:ci_variables, :key, :string, null: false)
+    change_column(:ci_variables, :project_id, :integer, null: false)
+  end
+
+  def down
+    # no op
+  end
+end