Build system, dependency changes

[mike42]

Changes necessary to get build/test rolling again for next release.

• Upgrade development dependencies, possibly find modern alternative to php_codesniffer

        "phpunit/phpunit": "^6.5",
        "squizlabs/php_codesniffer": "^3.3.1",
        "phpbench/phpbench": "@dev"

• Raise minimum PHP version for next release
• Write GitHub actions build to replace Travis build
• Replace build status badge in repo