Bump Jackson version a bit. (#2658)

This avoids a vulnerability on older 2.7 versions.

pom.xml

@@ -61,7 +61,7 @@
     <!-- Converter Dependencies -->
     <gson.version>2.8.2</gson.version>
     <protobuf.version>3.0.0</protobuf.version>
-    <jackson.version>2.7.2</jackson.version>
+    <jackson.version>2.8.0</jackson.version>
     <wire.version>2.2.0</wire.version>
     <simplexml.version>2.7.1</simplexml.version>
     <moshi.version>1.5.0</moshi.version>