deps: update dependency junit:junit to v4.13 (#62)

Author: renovate-bot

pom.xml

@@ -158,7 +158,7 @@
  <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>
  <opencensus.version>0.24.0</opencensus.version>