We're going to make it dynamic using the geozones. Besides, class
methods can be overwritten using custom models, while constants can't be
overwritten without getting a warning [1].
Makes the definition of segments with geozones a little cleaner. I
think it’s worth it, compared to the slight memory gain of using a
constant [2].
[1] warning: already initialized constant UserSegments::SEGMENTS
[2] https://stackoverflow.com/questions/15903835/class-method-vs-constant-in-ruby-rails#answer-15903970
