Modernize build automation

[sebastianbergmann]

• Use GitHub Actions instead of Travis CI
• Have Composer in tools/composer and managed through composer self-update (see update-tools target in build.xml
• Install Psalm using Phive as tools/psalm (phive install --copy psalm)
• Install PHP-CS-Fixer using Phive as tools/php-cs-fixer (phive install --copy php-cs-fixer)
• Create Psalm configuration
• Add Psalm build step to GitHub Actions-based CI workflow
• Add PHP-CS-Fixer build step to GitHub Actions-based CI workflow