CI: avoid flaps with Koala.configure

[ylecuyer]

Before: we run Koala.reset_config in a before(:each) to avoid mutation on the global Koala.configuration object.

But there are some specs (for example ./spec/cases/test_users_spec.rb:285) which have their own before(:each) making calls to the graph api (before the reset) and in some cases it conflicts with the current state of the Koala.configuration for example after a test setting a bad configuration on purpose

Proposed fix is to reset the configuration in an after(:each) instead