Update PowerMock to 2.0.2

and set Mockito 2's mock-maker-inline as MockMaker to allow mocking of
final classes.

build.gradle

@@ -130,7 +130,7 @@ allprojects {
 			':smack-openpgp',
 		].collect { project(it) }
 		junitVersion = '5.4.2'
-		powerMockVersion = '1.7.3'
+		powerMockVersion = '2.0.2'
 	}
 	group = 'org.igniterealtime.smack'
 	sourceCompatibility = JavaVersion.VERSION_1_8

smack-extensions/src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker

@@ -0,0 +1 @@
+mock-maker-inline