Update to 0.8.1

• Switch to depending on upstream tarballs for all the JAR files this port needs. Since 0.8.0, the upstream switched to a Kotlin-based Gradle file, which broke the Gradle dependency resolution mechanism implemented in files/build.gradle.in. It is unproductive to fix that script. Let's just use the provided JAR files.
• Java is no longer used to during build, so set JAVA_RUN=jre.
• Release notes: https://github.com/AsamK/signal-cli/blob/v0.8.1/CHANGELOG.md