Fix licence info generation in Maven POM

The currently published POMs are invalid according to the XSD, and are
rejected by some artifact managers (e.g. Artifactory). The licence info is
sitting directly inside the <licenses> tag, rather than in a nested
<license> tag.

The builder notation is picking up the licence plugin object rather than
generating a new property. This fixes that by referencing delegate.licence
instead.
Merge pull request #8 from tomdcc/fix-pom-licence

Fix licence info generation in Maven POM