improve makefile (use latexmk)

1 job for master in 4 minutes and 54 seconds (queued for 34 seconds)
Name Stage Failure
build:pdf file Build
  latex: Command for 'latex' gave return code 1
Refer to 'slides.log' for details
Latexmk: Use the -f option to force complete processing,
unless error was exceeding maximum runs, or warnings treated as errors.
=== TeX engine is 'pdfTeX'
Latexmk: Errors, so I did not complete making targets
make: *** [Makefile:5: slides.pdf] Error 12
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1