title: artillery.io JSON value capture not working
body: <p>Using Artillery.io 1.6.0-10, I call an api that returns a JSON and try to capture one of the values for later use in the flow, however the capture doesn't seem to be working. Here is the simplified code:</p>
  
  <p>get_ddg.yml</p>
  
  <pre><code>config:
    target: "https://api.duckduckgo.com"
    phases:
      - duration: 3
        arrivalCount: 1
  scenarios:
    - name: "Get search"
      flow:
        - get:
            url: "/?q=DuckDuckGo&amp;format=json"
            capture:
              json: "$.Abstract"
              as: "abstract"
        - log: "Abstract: {{ $abstract }}"
  </code></pre>
  
  <p>When I run artillery the value is empty:</p>
  
  <pre><code>$ artillery run get_ddg.yml
  Started phase 0, duration: 3s @ 10:28:34(+0200) 2017-10-25
  ⠋ Abstract:   &lt;----- EMPTY! NO VALUE FOR $abstract
  Report @ 10:28:37(+0200) 2017-10-25
  Scenarios launched:  1
  Scenarios completed: 1
  Requests completed:  1
  Concurrent users:    1
  RPS sent: 2.08
  Request latency:
    min: 311.9
    max: 311.9
    median: 311.9
    p95: NaN
    p99: NaN
  Scenario duration:
    min: 349.5
    max: 349.5
    median: 349.5
    p95: NaN
    p99: NaN
  Codes:
    200: 1
  </code></pre>
  
  <p>Any help is much appreciated.</p>