title: Error when trying to optimize with Portfolio Analytics
body: <p>I'm trying to copy the code from a website to test Portfolio Analytics library in R. But I getting an error and I don't have a clue why. The error I am getting is: Error: "package:ROI" %in% search() || requireNamespace("ROI", quietly = TRUE) is not TRUE</p>
  
  <pre><code>library(PortfolioAnalytics)
  
  data(edhec)
  returns &lt;- edhec[, 1:6]
  funds &lt;- colnames(returns)
  
  init.portfolio &lt;- portfolio.spec(assets = funds)
  
  init.portfolio &lt;- add.constraint(portfolio = init.portfolio, 
                                   type = "full_investment")
  
  init.portfolio &lt;- add.constraint(portfolio = init.portfolio, 
                                   type = "long_only")
  
  minSD.portfolio &lt;- add.objective(portfolio=init.portfolio, 
                                   type="risk", 
                                   name="StdDev")
  
  minSD.opt &lt;- optimize.portfolio(R = returns, portfolio = minSD.portfolio, 
                                  optimize_method = "ROI", trace = TRUE)
  </code></pre>