Updates to parent pom

[Carsten Ziegeler <cz...@apache.org>]

Hi,

as outlined in FELIX-5820 I've done some updates/changes to our parent
pom. This is the list of changes:

- update to latest Apache parent pom
- set java min version to 7
- reduce supported java versions to 6, 7, 8, 9 (drop 4, 5, add 8, 9)
- add missing encoding properties
- remove dependency management of testing dependencies (like junit,
easymock etc.) as these easily get outdated and this section does not
really provide value
- update versions of plugins
- use enforcer plugin to enforce maven and min java version
- remove calculation of BREE header as this one is deprecated, and bnd
does the capability calculation anyway

Please let me know if there is anything else to be done before we can
cut a release

Thanks
Carsten
-- 
Carsten Ziegeler
Adobe Research Switzerland
cziegeler@apache.org