Add step to appveyor process to cancel builds on PR update (#1239)
Add step to appveyor process to cancel builds on PR update (#1239)

Right now when we're queuing jobs for appveyor ci if we push updates to
the same PR we still run the jobs for both the outdated revision of the
PR (which we throw away) and then the new revision. During periods of
active or fast paced development this can lead to very long queues. This
commit partially addresses the problem by canceling jobs if a PR update
has been pushed. While it won't solve the complete problem it does
improve the situation.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py