Update/split import-sources to make it compatible with Maven 3.1

http://jira.codehaus.org/browse/MNG-5214 is fixed in Maven 3.1, which made
src/it/import-sources fail (notably on Travis CI). The test is now split
in two parts, one compatible with Maven 3.1, the other one only compatible
with Maven 3.0 (and only run when the proejct is built with Maven 3.0)