zinc compiler does not honor "force" option

When setting the force option on a ScalaCompile task using the Zinc compiler:

tasks.withType(ScalaCompile) {
    scalaCompileOptions.with {
        useAnt = false
        force = true
    }
}

A full recompile does not occur and it just performs another incremental compile.

This issue is basically just tracking a fix from https://github.com/gradle/gradle/pull/385.