Package ‘XML’ is not available when running with R version 3.6.3

Hi, I’m trying to get Travis setup for my R package. I’m new with Travis might be something very simple! My .travis.yml is very basic:

language: R
r: oldrel
cache: packages

Am trying to get it working with the old release of R (3.6.3) but the latest version of the XML package (which is a dependency) needs R>4.0. As a result I get the following error:

*     DONE (sctransform)
Installing 105 packages: xfun, rlang, base64enc, digest, stringi, magrittr, glue, markdown, highr, stringr, tinytex, mime, jsonlite, evaluate, htmltools, yaml, knitr, rmarkdown, gtable, Rcpp, plyr, listenv, globals, future, iterators, colorspace, utf8, assertthat, vctrs, pkgconfig, pillar, lifecycle, fansi, ellipsis, crayon, cli, tibble, rematch, ps, processx, backports, prettyunits, callr, rstudioapi, rprojroot, pkgbuild, desc, praise, pkgload, viridisLite, RColorBrewer, R6, munsell, labeling, farver, testthat, withr, scales, isoband, sys, askpass, bit, plogr, BH, memoise, bit64, openssl, generics, blob, tidyselect, purrr, httr, curl, rappdirs, DBI, RSQLite, dplyr, dbplyr, hms, S4Vectors, IRanges, Biobase, BiocGenerics, BiocManager, bookdown, RcppEigen, gridExtra, reshape2, ggplot2, future.apply, foreach, abind, progress, cellranger, BiocFileCache, AnnotationDbi, XML, BiocStyle, RNOmni, ggdendro, HGNChelper, readxl, cowplot, limma, biomaRt
Installing packages into ‘/home/travis/R/Library’
(as ‘lib’ is unspecified)
Error: (converted from warning) package ‘XML’ is not available (for R version 3.6.3)
Execution halted
The command "Rscript -e 'deps <- remotes::dev_package_deps(dependencies = NA);remotes::install_deps(dependencies = TRUE);if (!all(deps$package %in% installed.packages())) { message("missing: ", paste(setdiff(deps$package, installed.packages()), collapse=", ")); q(status = 1, save = "no")}'" failed and exited with 1 during .
Your build has been stopped.

Here is the build report

Could it be because the package which has XML as a dependency is installed from github?

Any suggestions on how to get this working would be very much welcomed!