Using fakeweb only if flag is given


I’m using the gem fakeweb to simulate the comunication between my
ruby-app and different other apps when running my rspecs. The advantage
is, that the specs work much faster and it is not nessessary that the
other apps are currently avalable. But for testing the integration of my
app, it is of course nessessary to check weather my component is
compatible with the others. What im searching is something like a flag
to the comment bundle exec rspec spec e.g. bundle exec rspec spec --fake
do decide to run fakeweb or not. Does something like this exist and if
so how can I realize this.

Best regards