Specify version 2.4 for the maven-jar-plugin.

Got the following warning from maven when installing:

[WARNING]
[WARNING] Some problems were encountered while building the effective model for se.lth.cs.nlp:wikiforia:jar:1.0-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-jar-plugin is missing. @ line 38, column 15
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
Generate sources by default.
Merge pull request #2 from simonlindberg/master

Generate source jar by default and use specific the maven-jar-plugin version.