Travis fixes

[bockthom]

Prerequisites

• I adhere to the coding conventions (described here) in my code.
• I have updated the copyright headers of the files I have modified.
• I have written appropriate commit messages, i.e., I have recorded the goal, the need, the needed changes, and the location of my code modifications for each commit. This includes also, e.g., referencing to relevant issues.
• I have put signed-off tags in all commits.
• I have updated the changelog file NEWS.md appropriately.
• The pull request is opened against the branch dev.

Description

Adjust package installation script to fix Travis CI builds:

Travis builds currently fail (for a few weeks) due to two different reasons:

On the one hand, the log output exceeds the maximum log length of travis,
resulting in failing builds. This is prevented by setting the parameters
`verbose` and `quiet` to `TRUE`, as then the package compilation log is
not printed to the log but the result of package installation is clearly
perceptible in the log.

On the other hand, travis installs lots of packages which are not necessary
as they are just suggestions of other packages but not necessary dependencies.
Installing such unnecessary packages my lead to problems. This is
circumvented by setting the `dependencies` parameter to `NA` as this results
in installing only necessary dependencies and imports, whereas the former
`TRUE` value of this parameter also led to installing suggested but
unnecessary packages.

Those two fixes make the Travis CI builds successful again.

Props to @clhunsen for experimenting with the parameters and proposing
the first part of this commit regarding the `verbose` and `quiet`
parameters of the `install.packages` function.

In addition, add R version 4.0 to the test suite.

Changelog

• Adjust package installation in install.R to improve compatibility with Travis CI (1ba0367, R versions to support and which to cover in test suite #161)
• Adjust recommended R version to 3.6.3 in README (92be262)
• Add R version 4.0 to test suite (40aa0d8, R versions to support and which to cover in test suite #161)

[clhunsen]

Thank you very much for your fixes, @bockthom! Everything looks nice and feels cozy. 😀 👍

@hechtlC, you can merge if you agree. 🙂

[hechtlC]

Nice work thanks @bockthom .
I will merge this now.