ARROW-1242: [JAVA] - upgrade jackson to mitigate security vulnerabilities (take 2)

sorry, PR #929 failed to actually change the Jackson version, since the `jackson.version` variable defined in java/pom.xml is not used in java/vector/pom.xml

That's now fixed in this PR.

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

Closes #957 from mattdarwin/ARROW-1242-upgrade-jackson and squashes the following commits:

ad15e5f [Matt Darwin] Merge branch 'master' into ARROW-1242-upgrade-jackson
ee29d65 [Matt Darwin] Merge branch 'master' of https://github.com/apache/arrow into ARROW-1242-upgrade-jackson
06d7745 [Matt Darwin] upgrading jackson to 2.7.9 PROPERLY this time...
284a4ce [Matt Darwin] Merge branch 'master' of https://github.com/apache/arrow
d059517 [Matt Darwin] 1242 upgraing jackson to 2.7.9
bc3b6a0 [Matt] Merge pull request #1 from apache/master

Author: Matt Darwin

java/vector/pom.xml

@@ -39,12 +39,12 @@
     <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>
       <artifactId>jackson-annotations</artifactId>
-      <version>2.7.1</version>
+      <version>${jackson.version}</version>
     </dependency>
     <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>
       <artifactId>jackson-databind</artifactId>
-      <version>2.7.1</version>
+      <version>${jackson.version}</version>
     </dependency>
     <dependency>
       <groupId>com.carrotsearch</groupId>