The activity method occurs `NameError`

[sinsoku]

When I call client.activity at first, it occurs NameError: uninitialized constant TrackerApi::Resources::Activity::Virtus.

Reproduce

$ rake test TEST=test/client_test.rb TESTOPTS='--name="TrackerApi::Client::.activity#test_0001_gets all my activities"'
[Coveralls] Set up the SimpleCov formatter.
[Coveralls] Using SimpleCov's default settings.
Run options: "--name=TrackerApi::Client::.activity#test_0001_gets all my activities" --seed 24196

# Running:

E

Finished in 0.020668s, 48.3845 runs/s, 0.0000 assertions/s.

  1) Error:
TrackerApi::Client::.activity#test_0001_gets all my activities:
NameError: uninitialized constant TrackerApi::Resources::Activity::Virtus
    /Users/tshotoku/src/tracker_api/lib/tracker_api/resources/activity.rb:4:in `<class:Activity>'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/resources/activity.rb:3:in `<module:Resources>'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/resources/activity.rb:2:in `<module:TrackerApi>'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/resources/activity.rb:1:in `<top (required)>'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/endpoints/activity.rb:15:in `require'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/endpoints/activity.rb:15:in `block in get'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/endpoints/activity.rb:14:in `map'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/endpoints/activity.rb:14:in `get'
    /Users/tshotoku/src/tracker_api/lib/tracker_api/client.rb:203:in `activity'
    /Users/tshotoku/src/tracker_api/test/client_test.rb:214:in `block (4 levels) in <top (required)>'
    /Users/tshotoku/.rbenv/versions/2.5.0-dev/lib/ruby/gems/2.5.0/gems/vcr-3.0.3/lib/vcr/util/variable_args_block_caller.rb:9:in `call_block'
    /Users/tshotoku/.rbenv/versions/2.5.0-dev/lib/ruby/gems/2.5.0/gems/vcr-3.0.3/lib/vcr.rb:189:in `use_cassette'
    /Users/tshotoku/src/tracker_api/test/client_test.rb:213:in `block (3 levels) in <top (required)>'

1 runs, 0 assertions, 0 failures, 1 errors, 0 skips