[portlet] Update parent pom reference to latest TP based on Neon

As Max pointed out, there are no changes in code base and in JBoss Tools p2 repository assembly we use latest released portlet version, but we should build it against latest Eclipse Neon in TP to catch at least compilation problems (if any) related to new Eclipse release stream.

It is required, because portlet is available though JBoss Central in nightly builds from master and we have to be sure it is usable.