Bump Jackson dependency to 2.9.8

Addresses a number of CVEs that affect versions from 2.9.0 through 2.9.7,
inclusive.

Author: michaelklishin

pom.xml

@@ -57,7 +57,7 @@
     <slf4j.version>1.7.25</slf4j.version>
     <metrics.version>3.2.6</metrics.version>
     <micrometer.version>1.0.2</micrometer.version>
-    <jackson.version>2.9.6</jackson.version>
+    <jackson.version>2.9.8</jackson.version>
     <httpclient.version>4.5.6</httpclient.version>
     <logback.version>1.2.3</logback.version>
     <junit.version>4.12</junit.version>