Reimplement lazy evaluation of aspectjVersion

Since Providers are not supported as dependency notation in Gradle 5.x
we now evaluate the aspectjVersion in the afterEvaluate callback.

Fixes #4

src/main/groovy/aspectj/AspectjGradlePlugin.groovy

@@ -29,11 +29,15 @@ class AspectjGradlePlugin implements Plugin<Project> {
 		project.plugins.apply(JavaPlugin)
 
 		if (project.configurations.findByName('ajtools') == null) {
-			def aspectjVersion = project.objects.property(String).convention(project.provider { project.aspectjVersion })
-			project.configurations.create('ajtools')
-			project.dependencies {
-				ajtools aspectjVersion.map { "org.aspectj:aspectjtools:${it}" }
-				compile aspectjVersion.map { "org.aspectj:aspectjrt:${it}" }
+			project.afterEvaluate {
+				if (!project.hasProperty('aspectjVersion')) {
+					throw new GradleException("You must set the property 'aspectjVersion' before applying the aspectj plugin")
+				}
+				project.configurations.create('ajtools')
+				project.dependencies {
+					ajtools "org.aspectj:aspectjtools:${project.aspectjVersion}"
+					compile "org.aspectj:aspectjrt:${project.aspectjVersion}"
+				}
 			}
 		}