ARROW-1242: [JAVA] - upgrade jackson to mitigate security vulnerabilities

As per apache#872 I am upgrading Jackson to the latest version on the current train (2.7.1 --> 2.7.9)

Author: Matt Darwin <(none)>
Author: Matt <mattdarwin@yahoo.co.uk>

Closes apache#929 from mattdarwin/ARROW-1242-upgrade-jackson and squashes the following commits:

d059517 [Matt Darwin] 1242 upgraing jackson to 2.7.9
bc3b6a0 [Matt] Merge pull request #1 from apache/master

Author: Matt Darwin

java/pom.xml

@@ -33,7 +33,7 @@
     <dep.slf4j.version>1.7.25</dep.slf4j.version>
     <dep.guava.version>18.0</dep.guava.version>
     <forkCount>2</forkCount>
-    <jackson.version>2.7.1</jackson.version>
+    <jackson.version>2.7.9</jackson.version>
     <hadoop.version>2.7.1</hadoop.version>
     <checkstyle.failOnViolation>false</checkstyle.failOnViolation>
   </properties>