[CI] Test R vignettes

[hcho3]

One R vignette is failing because it didn't get updated with the codebase.

--- re-building ‘xgboostPresentation.Rmd’ using rmarkdown
Loading required package: xgboost
Quitting from lines 412-424 (xgboostPresentation.Rmd)
Error: processing vignette 'xgboostPresentation.Rmd' failed with diagnostics:
[01:06:51] amalgamation/../src/learner.cc:506: Check failed: mparam_.num_feature != 0 (0 vs. 0) : 0 feature is supplied.  Are you using raw Booster interface?
Stack trace:
  [bt] (0) /home/phcho/Desktop/xgboost/xgboost.Rcheck/xgboost/libs/xgboost.so(dmlc::LogMessageFatal::~LogMessageFatal()+0x7c) [0x7f33473da05c]
  [bt] (1) /home/phcho/Desktop/xgboost/xgboost.Rcheck/xgboost/libs/xgboost.so(xgboost::LearnerConfiguration::ConfigureNumFeatures()+0xeb) [0x7f33474932ab]
  [bt] (2) /home/phcho/Desktop/xgboost/xgboost.Rcheck/xgboost/libs/xgboost.so(xgboost::LearnerConfiguration::Configure()+0x3ba) [0x7f33474d363a]
  [bt] (3) /home/phcho/Desktop/xgboost/xgboost.Rcheck/xgboost/libs/xgboost.so(xgboost::LearnerImpl::Predict(std::shared_ptr<xgboost::DMatrix>, bool, xgboost::HostDeviceV
ector<float>*, unsigned int, bool, bool, bool, bool, bool)+0x78) [0x7f3347491b98]
  [bt] (4) /home/phcho/Desktop/xgboost/xgboost.Rcheck/xgboost/libs/xgboost.so(XGBoosterPredict+0xef) [0x7f33473ef86f]

[hcho3]

These lines should be removed:

xgboost/tests/ci_build/build_test_rpkg.sh

Lines 20 to 21 in e726dd9

	--no-vignettes \
	--no-manual \

[trivialfis]

I think we have checks now. (--as-cran)