Upgrade to Test Retry Gradle Plugin 1.6.0

Closes gh-41

build.gradle

@@ -9,7 +9,7 @@ buildscript {
     dependencies {
         classpath "io.github.gradle-nexus:publish-plugin:2.0.0"
         classpath "org.graceframework:grace-gradle-plugin:$graceVersion"
-        classpath "org.gradle:test-retry-gradle-plugin:1.4.0"
+        classpath "org.gradle:test-retry-gradle-plugin:1.6.0"
         classpath "io.spring.gradle:dependency-management-plugin:$springDependencyManagementPluginVersion"
     }
 }