> Another requirement further qualifies the required detriment component; the promisee must have suffered an actual substantial detriment in the form of an economic loss that results from the promisor failing to deliver on his or her promise. Finally, promissory estoppel is usually only granted if a court determines that enforcing the promise is essentially the only means by which injustice to the promisee can be rectified.
The open source project would not suffer an economic loss by not using Travis. They would be free to stop using Travis or go to another competitor providing a free plan.
Exceptions to the above I can think of:
* Travis didn't charge them, then started charging them despite the promise
* they suffer an unrelated economic loss because they stop using CI
* they're forced to hire someone to convert them to another CI system (economic loss in form of wages, money to operate their own CI), though the loss would have t obe substantial (is that relative? e.g. in case of a low budget, is even a small sum substantial?)