add java mail dependency

Otherwise we get a NoClassDefError for javax.mail.MessagingException at startup.

```
Caused by: java.lang.ClassNotFoundException: javax.mail.MessagingException
        at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1483) ~[catalina-6.0.24.jar:na]
        at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1329) ~[catalina-6.0.24.jar:na]
```