Update SLF4J and JavaMail dependencies

The groupId for JavaMail changed, see this stackoverflow answer from the lead developer of JavaMail.
So the new maven coordinates right now are:

<dependency>
    <groupId>com.sun.mail</groupId>
    <artifactId>javax.mail</artifactId>
    <version>1.5.5</version>
</dependency>

The SLF4J dependency also got a bit stale, the most recent version right now is:

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.13</version>
</dependency>

Thanks a lot for this library, it really makes my life easier. :)