Bump logback-core from 1.2.8 to 1.2.9 in /zipkin (#265)

Bumps logback-core from 1.2.8 to 1.2.9.

---
updated-dependencies:
- dependency-name: ch.qos.logback:logback-core
  dependency-type: direct:development
...

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zipkin/pom.xml

@@ -93,7 +93,7 @@
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-core</artifactId>
-            <version>1.2.8</version>
+            <version>1.2.9</version>
             <scope>test</scope>
         </dependency>
         <dependency>