Add support for npx

[codewithtyler]

It would be great if the task used npx for running newman so that newman doesn't have to be installed on the server running the collection.

[carlowahlstedt]

Adding your comment from gitter:
support for this should be fairly straightforward. Do a check to make sure npm version is at >= 5.2.0. If it's not then throw an error. If the server does have 5.2.0 installed then instead of checking for the path to newman just use npx newman