Build for Java 8; upgrade dependencies and plugins

In this release, I finally dropped support for Java 7. By building this library for Java 8, I'm able to upgrade dependencies to newer versions that resolve identified vulnerabilities.

NOTE: I was unable to update to the latest slf4j-parent release because the associated slf4j-api test JAR appears to lack corresponding updates to the test code, which results in a reproducible unit test failure.

Upgrade/optimize dependencies and plugins

This release contains no functional changes, but is instead focused on project housekeeping.

Publish an official release

This release marks the fourth year of the publication of the first artifacts from this project. Better late than never, I suppose.

In this release (in which I'm including the previous tag):

• Upgrade to the latest release of JUnit 4 (4.13.1)
• Switch from old-school Java version specification to Maven Toolchain
• Declare plug-in versions in a pluginManagement section of the POM
• Replace the deprecated version badge in README with a newer one