Fix repo processing crash when labels contained an integer

[bmesuere]

This pull request fixes an issue that crashed exercise repository processing when the labels contained an integer. All elements in labels are now first converted to a string before downcasing.

The error we received was

A NoMethodError occurred in repositories#reprocess:

  undefined method `downcase' for 1339:Integer

    hash['labels'] = hash['labels'].map(&:downcase).uniq if hash.key? 'labels'
                                   ^^^^
  app/models/activity.rb:503:in `map'

This issue was encountered by @bmoelans.