Mention the built-in alternative to Timecop for Rails

[pirj]

While still useful outside of Rails, Timecop has its downsides, one is that it's not thread-safe.

AS already has what Timecop provides, freeze_time, travel, travel_back, travel_to.

[andyw8]

@pirj I've suggested this as a cop for rubocop-rails:

rubocop/rubocop-rails#20

[sambostock]

FYI I've opened rubocop/rubocop-rails#38 to address this.