Support building on newer JDKs

[l0s]

This updates the CI configuration to build using JDKs 8, 10, and 11. Locally, I tested using 8, 9, 10, 12, and 14. The build issues were resolved by upgrading test and plugin dependencies and making minor code changes.

Resolves: #13

[codecov-commenter]

Codecov Report

Merging #14 (0983e08) into master (437d4d3) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #14   +/-   ##
=========================================
  Coverage     99.16%   99.16%           
  Complexity       37       37           
=========================================
  Files             4        4           
  Lines           120      120           
  Branches         10       10           
=========================================
  Hits            119      119           
  Partials          1        1           

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[l0s]

The CI server failures were also encountered and fixed in scope of #12 . I can port those changes here if desired.

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[RavenVSS]

Hi, @l0s Please resolve conflicts.

[l0s]

@RavenVSS, I resolved the merge conflicts. I also moved the build configuration from Travis to GitHub Actions and used the current list of supported JDK versions.

Two of the checks are failing. The SonarCloud check appears to be a configuration error and is also failing on master. The other requires certain labels on the PR, which will need to be added by a project maintainer.