Use throw :abort in callbacks in tests on Rails >5
Use throw :abort in callbacks in tests on Rails >5

This fixes a deprecation warning in tests, which explains the issue well

DEPRECATION WARNING: Returning `false` in Active Record and Active Model callbacks will not implicitly halt a callback chain in Rails 5.1. To explicitly halt the callback chain, please use `throw :abort` instead. (called from move_to at /home/travis/build/collectiveidea/awesome_nested_set/lib/awesome_nested_set/model/movable.rb:102)

This commit calls throw :abort instead when testing on Rails >= 5, which
fixes the deprecation warning on Rails 5.0 and fixes two test failures
on Rails 5.1

collectiveidea/awesome_nested_set,experteer/awesome_nested_set