Disable Grgit on 6.

It appears to be incompatible.

build.gradle

@@ -28,7 +28,7 @@ buildscript {
     dependencies {
         classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.3'
         classpath 'org.jfrog.buildinfo:build-info-extractor-gradle:3.0.1'
-        classpath 'org.ajoberstar:gradle-git:0.12.0'
+        classpath 'org.ajoberstar:gradle-git:1.4.2'
     }
 }
 
@@ -36,7 +36,7 @@ if (!project.hasProperty("artifactory_contextUrl")) ext.artifactory_contextUrl =
 if (!project.hasProperty("artifactory_user")) ext.artifactory_user = "guest"
 if (!project.hasProperty("artifactory_password")) ext.artifactory_password = ""
 
-if (!project.hasProperty("gitCommitHash")) {
+if (!project.hasProperty("gitCommitHash") && !JavaVersion.current().isJava6()) {
     try {
         def repo = org.ajoberstar.grgit.Grgit.open(project.file('.'))
         ext.gitCommitHash = repo.head().abbreviatedId