pom.xml: use 2.15-SNAPSHOT

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9390 5f5364db-9458-4db8-a492-e30667be6df6
JavaScript: handle RegExp constructed with 'undefined'.
Issue 1599

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9391 5f5364db-9458-4db8-a492-e30667be6df6