Support Java 8 language features in Java modules.

bramble-api/build.gradle

@@ -1,6 +1,6 @@
 apply plugin: 'java'
-sourceCompatibility = 1.6
-targetCompatibility = 1.6
+sourceCompatibility = 1.8
+targetCompatibility = 1.8
 
 apply plugin: 'witness'
 
@@ -35,3 +35,13 @@ task jarTest(type: Jar, dependsOn: testClasses) {
 artifacts {
 	testOutput jarTest
 }
+
+// If a Java 6 JRE is available, check we're not using any Java 7 or 8 APIs
+tasks.withType(JavaCompile) {
+	doFirst {
+		def home = System.env.JAVA_6_HOME;
+		if (home != null && !home.isEmpty()) {
+			options.bootstrapClasspath = files("${home}/jre/lib/rt.jar", "${home}/jre/lib/jsse.jar")
+		}
+	}
+}