Upgrade JUnit to 4.13 to allow using assertThrows

Test: ./gradlew test
Change-Id: I03714d1a20c6e8906fcca368cc684246add6555e
diff --git a/build.gradle b/build.gradle
index 5029a59..12c0d32 100644
--- a/build.gradle
+++ b/build.gradle
@@ -20,7 +20,7 @@
 
 dependencies {
     implementation 'com.google.protobuf:protobuf-javalite:3.8.0'
-    testImplementation 'junit:junit:4.12'
+    testImplementation 'junit:junit:4.13'
 }
 
 protobuf {