deps: update dependency junit:junit to v4.13 (#22)

Author: renovate-bot

pom.xml

@@ -69,7 +69,7 @@
     <gax.version>1.52.0</gax.version>
     <grpc.version>1.26.0</grpc.version>
     <protobuf.version>3.11.1</protobuf.version>
-    <junit.version>4.12</junit.version>
+    <junit.version>4.13</junit.version>
     <guava.version>28.1-android</guava.version>
     <threeten.version>1.4.0</threeten.version>
     <javax.annotations.version>1.3.2</javax.annotations.version>