make the project compatible with assertj-core:3.15.0

[Simulant87]

I have a project setup with this annotation-validator 2.0 and an explicit dependency to org.assertj:assertj-core:3.15.0 so the annotation-validator also uses the upgraded version of the dependency.

Having a test case like this, fails:

	@Test
	public void activatedAnnotatedCorrect() throws Exception {
		validate()
				.only()
				.annotation(type(Column.class))
				.forField(getFieldByName("activated"));
	}

With the stacktrace:

java.lang.NoSuchMethodError: org.assertj.core.api.OptionalAssert.as(Ljava/lang/String;[Ljava/lang/Object;)Lorg/assertj/core/api/AbstractAssert;
at de.tolina.common.validation.AnnotationValidation.forClassOrMethodOrField(AnnotationValidation.java:163)
at de.tolina.common.validation.AnnotationValidation.forField(AnnotationValidation.java:142)