Update to 0.6.8

• Set LICENSE_FILE.
• Drop LIB_DEPENDS, libunix-java.so is no longer needed.
• Use ${GMAKE} to call Gmake.
• Bump JAVA_VERSION to 8+
• Update build.gradle.in to make it compatible with the recent Gradle versions.
• Drop unnecessary JAVA_OPTS from signal-cli.in.

Release notes:
https://github.com/AsamK/signal-cli/releases/tag/v0.6.8