Fix maven issues for gson.

Trying to resolve the following issue:

Failed to execute goal on project: Could not resolve dependencies for project: Failed to collect dependencies for [org.restlet.jse:org.restlet:jar:2.2-M5 (compile), org.restlet.jse:org.restlet.ext.gson:jar:2.2-M5 (compile)]: Failed to read artifact descriptor for com.google.code.gson:com.google.code.gson:jar:2.2.4: Could not transfer artifact com.google.code.gson:com.google.code.gson:pom:2.2.4 from/to maven1-java (http://download.java.net/maven/1): No connector available to access repository maven1-java (http://download.java.net/maven/1) of type legacy using the available factories WagonRepositoryConnectorFactory -> [Help 1]

I think it's because of the spelling (artefact vs artifact).