Add GitHub PAT environmental variable

[Will Beasley]

To avoid the following warning on Travis

$ Rscript -e 'remotes::install_github(c("r-lib/covr"))'
Using bundled GitHub PAT. Please add your own PAT to the env var `GITHUB_PAT`

Discussed in several places, including https://www.datacamp.com/community/tutorials/cd-package-docs-pkgdown-travis

Source of message: https://github.com/r-lib/devtools/blob/master/R/github.R

[Will Beasley]

Now successfully reads the token

$ Rscript -e 'remotes::install_github(c("r-lib/covr"))'
Using github PAT from envvar GITHUB_PAT

https://travis-ci.org/github/IQSS/dataverse-client-r/jobs/752167916#L873