title: how to solve "ssl certificate problem: unable to get local issuer certificate" in ubuntu 16.04
body: <p>I'm using <code>rvest</code> to scrape a website. I'm totally OK with other websites but this one is using another type of certificate I think. I've seen similar questions here and in github but no one could help me.
  Any help is appreciated. 
  My script is as follows:</p>
  
  <pre><code>url &lt;- "https://search.codal.ir/api/search/v1/q?PageNumber=1&amp;Symbol=%D9%81%D8%B3%D8%A7&amp;Subject=%20&amp;CompanyState=0&amp;LetterType=6&amp;TracingNo=-1&amp;LetterCode=%20&amp;FromDate=1395/01/01&amp;ToDate=%DB%B1%DB%B3%DB%B9%DB%B8/%DB%B0%DB%B3/%DB%B1%DB%B6&amp;AuditorRef=-1&amp;YearEndToDate=&amp;Publisher=false&amp;Mains=true&amp;Childs=false&amp;Audited=false&amp;NotAudited=true&amp;Length=-1&amp;Consolidatable=true&amp;NotConsolidatable=true&amp;CompanyType=1&amp;Category=1"
  
  data &lt;- fromJSON(url)[[3]]
  </code></pre>
  
  <p>and the error is:</p>
  
  <blockquote>
    <p>"Error in open.connection(con, "rb") : 
      SSL certificate problem: unable to get local issuer certificate"</p>
  </blockquote>