Migrate CI from Travis to GitHub Actions with modern Node matrix

[shekhei]

Context

The repository uses .travis.yml with Node 6–8 and Travis CI. Travis is deprecated for many projects, and the Node versions are long EOL.

Why this matters

Modern CI improves reliability, catches regressions on supported Node versions, and avoids reliance on outdated infrastructure.

Proposal

• Add a GitHub Actions workflow that installs GraphicsMagick and runs both test suites.
• Use a current Node matrix (e.g., 16/18/20) while keeping webpack2/4 tests working.

Tasks

• Create .github/workflows/ci.yml with matrix builds.
• Install GraphicsMagick (apt-get) in workflow.
• Run npm test (or both test scripts) in CI.
• Optionally remove or deprecate .travis.yml.