build: add dependency to gwt-elemental

Close #6

Author: javier-godoy

pom.xml

@@ -80,6 +80,12 @@
             <version>1.18.38</version>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>com.vaadin.external.gwt</groupId>
+            <artifactId>gwt-elemental</artifactId>
+            <version>2.8.2.vaadin2</version>
+            <scope>compile</scope>
+        </dependency>
         <dependency>
             <groupId>com.flowingcode.vaadin.addons.demo</groupId>
             <artifactId>commons-demo</artifactId>