do backstop init
then replace scenario with...
{
"label": "My Homepage",
"url": "index.html",
"hideSelectors": [],
"removeSelectors": [],
"selectors": [
"nav",
".jumbotron",
"body .col-md-4:nth-of-type(1)",
"body .col-md-4:nth-of-type(2)",
"body .col-md-4:nth-of-type(3)",
"footer"
],
"readyEvent": null,
"delay": 500,
"misMatchThreshold" : 0.1,
"onBeforeScript": "onBefore.js",
"onReadyScript": "onReady.js"
}