Change pom to use skipTests instead of maven.test.skip

When using maven.test.skip=true, the build fails due to an unresolved dependency on the test-jar

See the following pages:

http://maven.40175.n5.nabble.com/test-jar-dependency-not-found-when-tests-are-disabled-td450199.html
http://maven.apache.org/surefire/maven-surefire-plugin/examples/skipping-test.html