Require Java 8 for sablecc-runtime as well (forgot to update this)

sablecc-runtime/build.gradle

@@ -5,7 +5,7 @@ plugins {
 }
 
 java {
-  sourceCompatibility = JavaVersion.VERSION_1_7
+  sourceCompatibility = JavaVersion.VERSION_1_8
   withSourcesJar()
   withJavadocJar()
 }