JDBC and Tomcat

I am writing my first application with Vaadin. I am an experienced Java developer but new to Vaadin, Maven, Tomcat, …
 
My application is just a simple CRUD application which uses a JDBC driver to contact my PostgreSQL database.  I have learned that I can easily add this driver to my project by adding the following dependency to the pom.xml file:

<dependency>
  <groupId>org.postgresql</groupId>
  <artifactId>postgresql</artifactId>
  <version>9.4.1212</version>
</dependency

When I make a war file and run it on a Jetty (v9) server everything works fine but when running the same war file on a TomCat  (v8) server I get an error: “No suitable driver found for jdbc …”.
 
Someone who knows what I'm doing wrong?

Thanks in advance for your help

Thanks, this works for me