Error(s) in re-building vignettes:
  ...
--- re-building ‘Guide-to-CountVectorizer.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
--- finished re-building ‘Guide-to-CountVectorizer.Rmd’

--- re-building ‘Guide-to-TfidfVectorizer.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
trying URL 'http://cran.us.r-project.org/src/contrib/xgboost_1.3.1.1.tar.gz'
Content type 'application/x-gzip' length 966952 bytes (944 KB)
==================================================
downloaded 944 KB

* installing *source* package ‘xgboost’ ...
** package ‘xgboost’ successfully unpacked and MD5 sums checked
** using staged installation
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking Backtrace lib... 
checking for backtrace in -lexecinfo... no
checking endian... 
configure: creating ./config.status
config.status: creating src/Makevars
** libs
make: Fatal error in reader: Makevars, line 15: Unexpected end of line seen
Current working directory /tmp/RtmpmpaqzV/working_dir/RtmpOVaGjv/R.INSTALL2a4c30e828f0/xgboost/src
ERROR: compilation failed for package ‘xgboost’
* removing ‘/tmp/RtmpmpaqzV/RLIBS_5eca3c9116f0/xgboost’

The downloaded source packages are in
	'/tmp/RtmpmpaqzV/working_dir/RtmpeNaG8q/downloaded_packages'
Quitting from lines 171-183 (Guide-to-TfidfVectorizer.Rmd) 
Error: processing vignette 'Guide-to-TfidfVectorizer.Rmd' failed with diagnostics:
there is no package called 'xgboost'
--- failed re-building ‘Guide-to-TfidfVectorizer.Rmd’

--- re-building ‘introduction.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
trying URL 'http://cran.us.r-project.org/src/contrib/xgboost_1.3.1.1.tar.gz'
Content type 'application/x-gzip' length 966952 bytes (944 KB)
==================================================
downloaded 944 KB

* installing *source* package ‘xgboost’ ...
** package ‘xgboost’ successfully unpacked and MD5 sums checked
** using staged installation
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking Backtrace lib... 
checking for backtrace in -lexecinfo... no
checking endian... 
configure: creating ./config.status
config.status: creating src/Makevars
** libs
make: Fatal error in reader: Makevars, line 15: Unexpected end of line seen
Current working directory /tmp/RtmpmpaqzV/working_dir/RtmpjPaaEH/R.INSTALL42f0143713ac/xgboost/src
ERROR: compilation failed for package ‘xgboost’
* removing ‘/tmp/RtmpmpaqzV/RLIBS_5eca3c9116f0/xgboost’

The downloaded source packages are in
	'/tmp/RtmpmpaqzV/working_dir/Rtmptpaa7z/downloaded_packages'
Quitting from lines 171-180 (introduction.Rmd) 
Error: processing vignette 'introduction.Rmd' failed with diagnostics:
there is no package called 'xgboost'
--- failed re-building ‘introduction.Rmd’

SUMMARY: processing the following files failed:
  ‘Guide-to-TfidfVectorizer.Rmd’ ‘introduction.Rmd’

Error: Vignette re-building failed.
Execution halted