Consider dropping `is-promise` as a dependency

[jonkoops]

Currently this module depends on is-promise, which allows 'promise like' objects with a then() method to be interpreted as a promise, for example:

isPromise({then:function () {...}}); // true

This has been historically done to handle polyfilling Promises in environments where they were not yet fully supported (e.g. the Promise global is not available). Since this is no longer needed as promises are widely supported it might be time to drop this dependency and instead use a simple instanceof Promise check, which would only allow standardized promises.