Finish upgrading eclipse-jarsigner-plugin to 1.3.1

Commit 46b0f8a04 started that upgrade while missing this additional
change.

Change-Id: I34319d7006be13534497499e97536a8278906b1f
Signed-off-by: Marco Miller <marco.miller@ericsson.com>

pom.xml

@@ -330,7 +330,7 @@
         <plugin>
           <groupId>org.eclipse.cbi.maven.plugins</groupId>
           <artifactId>eclipse-jarsigner-plugin</artifactId>
-          <version>1.1.7</version>
+          <version>1.3.1</version>
         </plugin>
         <plugin>
           <groupId>org.eclipse.tycho.extras</groupId>