Upgrade to JUnit 4.13.1

Closes gh-231

Author: philwebb

pom.xml

@@ -46,7 +46,7 @@
 		<maven-plugin-api.version>${maven-core.version}</maven-plugin-api.version>
 		<maven-plugin-annotations.version>3.5</maven-plugin-annotations.version>
 		<mockito.version>2.8.47</mockito.version>
-		<junit.version>4.12</junit.version>
+		<junit.version>4.13.1</junit.version>
 		<javassist.version>3.21.0-GA</javassist.version>
 		<picocontainer.version>1.2</picocontainer.version>
 		<system-rules.version>1.16.0</system-rules.version>