Depend on JSON API plugin, not outdated transitive dependency (#160)

The emoji-java library has a transitive dependency on a 2017 version of
the JSON library.  The licensing of the 2017 version of the JSON library
is not as clear as the licensing of the most recent library version.

The most recent library version is available as a Jenkins API plugin.
Depend on the Jenkins JSON API plugin instead of bundling the old JSON
API jar file in the plugin.

pom.xml

@@ -76,6 +76,18 @@
       <groupId>com.vdurmont</groupId>
       <artifactId>emoji-java</artifactId>
       <version>5.1.1</version>
+      <exclusions>
+        <exclusion>
+          <groupId>org.json</groupId>
+          <artifactId>json</artifactId>
+        </exclusion>
+      </exclusions>
+    </dependency>
+    <!-- Depend on JSON API plugin rather than the transitive
+x         dependency of the old JSON API from emoji-java -->
+    <dependency>
+      <groupId>io.jenkins.plugins</groupId>
+      <artifactId>json-api</artifactId>
     </dependency>
     <dependency>
       <groupId>org.commonmark</groupId>