Fix gson module artifact descriptors

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).