Possibly latex error on updated packages

[stanstrup]

From @nilshoffmann

The travis build still complains about this: Package xcolor Warning: Incompatible color definition on input line 1816. GitHub actions is just complaining about deprecation in https://github.com/rformassspectrometry/metaRbolomics-book/runs/2167721661?check_suite_focus=true#step:6:834

[nilshoffmann]

@stanstrup I think travis and GitHub CI use different versions of Ubuntu / Debian as part of rocker/verse. The GitHub builder seems to work, so let's just ditch travis?

[stanstrup]

I already disabled travis. But my guess is that the error you get about xcolor is caused by different version of some R package. So I just wanted to keep a note here that something might need to be fixed before we can update to newer R/BioC/etc. We can also close and re-open if that happens.