title: caretEnsemble failing with is(all.models, "caretList") is not TRUE
body: <p>I've tried the code of zachmayer from <a href="https://gist.github.com/zachmayer/5152157" rel="nofollow">https://gist.github.com/zachmayer/5152157</a>, but I got an error "is(all.models, "caretList") is not TRUE", when running caretEnsemble command. Should I use caretList instead of just making a list of all the models? Thanks.</p>
  
  <pre><code>`#Train some models  
  model1 &lt;- train(X[train,], Y[train], method='gbm', trControl=myControl,
              tuneGrid=expand.grid(.n.trees=500, .interaction.depth=15,.shrinkage = 0.01))
  model2 &lt;- train(X[train,], Y[train], method='blackboost', trControl=myControl)
  ...
  #Make a list of all the models  
  all.models &lt;- list(model1, model2, model3, model4, model5, model6, model7, model8, model9, model10)
  
  #Make a greedy ensemble
   greedy &lt;- caretEnsemble(all.models, iter=1000L)
  Error: is(all.models, "caretList") is not TRUE`
  </code></pre>