.travis.yml; bugfix; Implementing workaround to new data.table release

[bokov]

… failing to install, from upstream: Rdatatable/data.table#3938

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR

[bokov]

Not completely sure, but I think what might be wrong with Appveyor is the dev R version apparently is bumped to 4.0 and those paths don't exist on the CRAN mirrors yet. I filed a ticket with @krlmlr, krlmlr/r-appveyor#167