Add jaxb-api as a dependency

This allows Java 9 to run S3Proxy.  Java 9 removed javax.xml which
previous Java versions included so adding the explicit dependency
allows both versions to work.  References #270.  References #280.

pom.xml

@@ -340,6 +340,11 @@
       <artifactId>logback-classic</artifactId>
       <version>1.2.3</version>
     </dependency>
+    <dependency>
+      <groupId>javax.xml.bind</groupId>
+      <artifactId>jaxb-api</artifactId>
+      <version>2.3.0</version>
+    </dependency>
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>